Said Manual will be available at the following electronic address: www.csv.go.cr, from the entry into force of this decree, where the document is backed up by the corresponding certified digital signature.
(Note from Sinalevi: The "Manual for the development of infrastructure projects from the perspective of road safety, in the formulation and execution of the pertinent public works contracted by the Ministry of Public Works and Transport and by the Costa Rican State" was extracted from the website of the Road Safety Council and is transcribed below:)
This manual is directed at all those professionals responsible for the planning, design, construction, management, and administration of roads, both urban and rural.
The document aims to be a guide to help address road safety problems effectively through engineering, for which it presents a series of measures at each stage of the development of a road infrastructure project.
Table of Contents Part I Introduction and Background 5 1 Introduction 5 2 The Problem 5 2.1 Global Data 5 2.2 Problems in Costa Rica 6 3. General Concepts on Road Safety 7 3.1 HAV System 7 3.2 Infrastructure and Road Safety 9 3.2.1 Principle of Quality 9 3.2.2 Principle of Spatial Consistency 10 3.2.3 Principle of Temporal Consistency 11 Part II Planning Stage 13 1 Introduction 13 2 Functional Classification of Roads 13 2.1 Primary Arterials 14 2.2 Secondary Distributors (minor arterials) 15 2.3 Local Distributors 16 2.4 Access Roads (local roads) 17 2.5 Pedestrian Areas 18 2.6 Cyclist Routes 19 3. Planning for Different Land Uses 20 3.1 Residential Areas 20 3.2 Industrial Areas 21 3.3 Commercial Areas 22 3.4 Recreation and Tourism Areas 23 Part III Design Stage 25 1 Introduction 25 2 Geometric Design Parameters 25 2.1 Design Speed 25 2.2 Sight Distance or Visual Distance 26 2.3 Stopping Sight Distance 26 2.4 Decision Distance 26 2.5 Passing Sight Distance 27 2.6 Friction 27 3 Horizontal Alignment 28 3.1 Radius of Curvature 28 3.2 Lane Width 29 3.3 Shoulders 29 4 Vertical Alignment 29 4.1 Ascending Grades 29 4.2 Descending Grades 29 4.3 Vertical Curves 30 4.3 Climbing Lanes 30 5 Combination of Horizontal and Vertical Curves 31 6 Design of Roadside Margins 32 6.1 Background 32 6.2 The Concept of Forgiving Roads 32 Part IV Construction Stage 35 1 Introduction 35 2 The Construction Process of a Road 35 2.1 Generalities 35 2.2 Fundamental Characteristics of Roads 35 3 Traffic Control at Work Sites 36 3.1 Introduction 36 3.2 Temporary Traffic Control Zones 37 3.3 Traffic Management Plans 37 3.4 Temporary Traffic Control Devices 38 3.5 Control Techniques 39 Part V Operation Stage 41 1 Introduction 41 2 Road Safety Inspections 41 3 Traffic Accident Management Plans 41 Part I Introduction and Background 1 Introduction This document presents the different stages involved in the development of a road infrastructure project, and some road safety-conscious design principles, so that the professionals involved and decision-makers have a tool to guide them on how to make the national road network safer.
The guide presented is oriented towards "accident prevention" through the incorporation of key factors related to road safety during the planning, design, construction, and operation of roads and road networks.
The preparation of this document was fundamentally based on the free compilation of knowledge contained in various bibliographic sources, and its adaptation to the needs of this guide.
2 The Problem The first recorded death related to a motor vehicle took place in London in 1896.
Since then, traffic accidents have claimed more than 30 million lives. Around the world, authorities of all countries are now aware of the number of people killed and injured on the roads, and of the social and economic consequences that this phenomenon generates.
As infectious diseases have been gradually controlled, deaths and injuries produced on the roads have gained importance in relation to other causes of mortality and disability. The World Health Organization (WHO) and the World Bank estimated that in 1990 traffic accidents occupied ninth place out of a total of 100 identified causes of death and disability (Murray and Lopez, 1996). For the year 2020, it is projected that traffic accidents will occupy second place in terms of "years of life lost", third place in terms of "disability-adjusted life years (DALYs) 1" and sixth place as a cause of death (Table I-1).
1 DALYs express years of life lost due to premature death, as well as years lived with a disability, adjusted for the severity of the disability.
2.1 Global Data Traffic accidents are a problem of great magnitude at the global level. International statistical data presented by the WHO highlight this problem:
More than 1.2 million people die each year due to road accidents.
Approximately 91% of traffic accidents resulting in fatal victims occur in low- and middle-income countries, which possess only 48% of the vehicle fleet registered worldwide.
Around 1,000 children and persons under 25 years of age die in traffic accidents daily, this being the leading cause of death among the 10 to 24 age group.
Young drivers, belonging to the male sex, are the most prone to committing infractions for speeding, this being consistent with one-third of the total traffic accidents in the world.
The use of the seat belt on the road can reduce the risk of death by 61% at the time of the incident. Also, using appropriate safety equipment for children inside the vehicle, such as child seats, can decrease their mortality by an order of 35%.
Include graphs-tables with figures for traffic accident deaths and vehicle fleet, distributed by country-region to show that lower-income countries concentrate the highest number of deaths, despite possessing fewer vehicle fleets.
According to TRRL (1991), the absence of systematic checks of the implications of road safety in new or rehabilitated road networks may be worsening the situation and ensures that "there are increasingly more unsafe roads, because extra special safeguards (normal in developed countries) are rarely incorporated into design processes to overcome operational inefficiencies." Comment on the policies, goals, and measures that have been taken in Europe and on the results they have obtained in reducing the figures for accidents and fatal accidents. Emphasize those aspects related to infrastructure.
2.2 Problems in Costa Rica Include in this section statistics prepared by the Road Safety Council.
Analyze the behavior of deaths in situ it is worth noting the number of deaths in situ. General downward trend in recent years (See Figure I-1), as is the case of 321 deaths in situ in 2004, going down to 277 in 2007. The figures and the graph in the Figure must be updated.
According to the type of accident, of the total deaths in situ occurring on national roads between 2004 and 2007 (Figure I-2), 28.3% correspond to pedestrian-vehicle collisions (atropellos), that is, accidents in which the victims were vulnerable users (pedestrians and cyclists). In said period, the number of cyclist deaths on national roads reached the number of 134 people, while pedestrians accounted for 341 fatalities in road accidents. The above places our country within the typical behavior of a developing country in terms of road safety, where the number of vulnerable victims (pedestrians, cyclists) exceeds the victims traveling inside the vehicle. 22.2% were accidents where vehicles ran off the road and 3.8% of accidents were rollovers. In all these cases, infrastructure with a more adequate design could have reduced the consequences of the accidents and avoided many of these deaths.
COSEVI figures show that the majority of vehicles involved in accidents in which people die in situ are of the sedan type, followed by motorcycles, which also represent vulnerable users. People between 20 and 25 years of age are the most exposed to this situation.
Road lighting and nighttime signage in our country is an important aspect to consider, since the majority of deaths in situ occur during nighttime and early morning hours. During the time slot from 6 p.m. to 6 a.m., in the period between 2004 and 2007, 716 deaths occurred, while during the day, between 6 a.m. and 6 p.m., the figure reached was 489 fatalities.
Comment on the observations that have been made in the ASV reports from Lanamme and the Report on the Florencio del Castillo by G. Valverde (2009), about the road safety conditions on the road margins. Refer to Figure I-3.
3. General Concepts on Road Safety 3.1 HAV System Most accidents cannot be attributed to a single cause, but are the result of a complex sequence of actions and interactions between several components of the human-environment-vehicle (HAV) system. The components can be a very effective strategy to solve a specific problem. This generates a synergy effect that increases the benefit obtained from individual actions. For example, the combination of modifications to traffic laws, actions in the field of education, and the promotion of police surveillance activities have been very useful to increase the use of seat belts and reduce the numbers of deaths from traffic accidents.
Therefore, safety problems must be addressed through the implementation of integral actions that take into account each of the components of the HAV system.
Because this manual is aimed at the implementation of road safety measures in the different stages of development of a road infrastructure project, greater emphasis will be given to the description of the infrastructure component (which forms part of the environment element in the HAV system). However, it must be clear that road safety concepts related to infrastructure design cannot be considered independently from the other two components of the system, the human factor and the vehicle.
Generally, the predominant role that the human component has in traffic accidents is highlighted. However, the fact that human factors are involved in the majority of accidents does not mean that only this component of the system should be addressed.
It must be taken into consideration that changes in human behavior are achieved very slowly and progressively. In contrast, infrastructure conditions can be modified and immediate results obtained.
The Venn diagram in Figure I-4 shows that significant benefits in road safety could be obtained by working on the interaction that exists between human factors and the environmental components of the infrastructure.
The HAV system can be represented using the Haddon matrix, which combines the three components of the system and the three phases of an accident (before, during, and after), as in Table I-2.
Professionals responsible for the development of road infrastructure must consider how the environmental factors of the road contribute to road safety during the three phases of a traffic accident, and effectively incorporate these elements within the execution of all stages of project development.
3.2 Infrastructure and Road Safety The road network must be appropriately planned and designed to achieve safe, efficient, and economical circulation of all users of the infrastructure, and therefore must minimize the consequences or the negative impact that such circulation can produce.
The road safety requirements of a road cannot be expressed in a simple manner. The factors that contribute to appropriate functioning as well as the conclusions obtained from the malfunctioning of the system must be taken into account.
The main responsibility of the authorities in charge of the national road network consists of taking actions on the environmental factors of the road. However, upon observing Figure I-4, it is easy to reach the conclusion that it is of primary importance to adequately consider the human-infrastructure interactions, a concept that can be termed "road ergonomics (ergonomía de la carretera)". The vehicle-infrastructure interactions, which focus on road design with geometric characteristics adequate for the behavioral dynamics of the vehicles and to provide ergonomic driving for the drivers, must also be considered.
In order to achieve safe traffic operation, road engineers and other professionals responsible for the development of road infrastructure projects must respect three basic principles:
The principle of quality: fully complying with five basic requirements:
visibility, roads with self-explaining design, adequacy of the infrastructure to the dynamics of the vehicles, maneuvering and recovery possibilities, reduction of impact severity.
The principle of spatial consistency:
complete consistency of all road elements with their surroundings, consistency of the road characteristics along the entire route.
The principle of temporal consistency:
planned road design.
3.2.1 Principle of Quality Five requirements must be satisfied:
Visibility It is estimated that about 90% of the information used in driving is visual. As a basic requirement, it must be ensured that the quality of the visual information contained in the surroundings of the road contributes to facilitating the driving task.
The driver must have the visual information in time, in such a way as to allow them to adapt their behavior safely to the situation they have encountered (based on the operating speeds).
Pedestrians and other users wishing to cross the road must be able to observe vehicles traveling on it from far enough away, to have the necessary time to process the information, decide when to cross, and complete their maneuver safely.
In France, the intersection sight distance is calculated using 8 seconds (with 6 seconds as an absolute minimum) at the operating speed V85 2 on the main road. On a 3-lane road or a divided 2-lane road, these values increase to 9 and 7 seconds, respectively (Service d´Études Technicques des Routes et Autoroutes, 1994).
2 V 85 Speed or lower, at which 85% of drivers travel.
Self-explaining roads The infrastructure and its surroundings must be easily understood by road users, in such a way that they can identify where they are, in which direction they must go, and be able to easily anticipate events that they will have to face - movements of vehicles and pedestrians, changes in the infrastructure, among others - with the purpose of adjusting their behavior accordingly (see Figure I-5).
Adequacy of the infrastructure to the dynamics of the vehicles Road characteristics must minimize the risk of dynamic failures of skidding, rollover, and others, according to the operating speed.
For example:
sudden changes in the horizontal radius of curvature, reduced friction coefficient at intersections, the existence of small hills on the road can promote high driving speeds (for example, on first-order rural roads).
skid marks on the road can destabilize motorcycles.
Maneuvering and recovery possibilities The infrastructure design must "forgive" driver errors, that is, it must allow drivers to maneuver and regain control in critical situations, avoiding running off the road and colliding with rigid objects, rolling over, hitting a third party, or falling over an embankment, etc. (See Figure I-6) Reduction of impact severity Obstacles located on the road margins must be at a sufficient distance, or not be so rigid, as to avoid serious consequences in an accident where the vehicle leaves the road. When this requirement cannot be satisfied, objects beside the road must be eliminated, moved, modified to make them frangible, or protected by means of vehicle restraint systems.
3.2.2 Principle of Spatial Consistency This consistency criterion cannot be considered independently. On the contrary, the operating speeds of the users must be taken into consideration, which is partially conditioned by the criterion of self-explaining road design. This leads to two consistency requirements:
Complete consistency of all road elements with their surroundings Examples of dangerous situations:
road with high-speed design characteristics, for example, divided roads, grade-separated intersections, but which has critical points, for example, access to private properties, narrow (or non-existent) shoulders, rigid obstacles near the travel lanes, among others.
residential streets with design characteristics that do not adapt to the presence of pedestrians and other non- motorized users (grid network, straight road alignment, wide travel lanes, among others).
Consistency along the entire route So that drivers can adapt their behavior safely, they must understand what type of road they are traveling on and predict the situations or conditions that await them ahead.
This requires defining a system of road categories in which each type of route is distinguished by a consistent set of design characteristics.
3.2.3 Principle of Temporal Consistency Road safety is strongly influenced by planned and unplanned changes in traffic flows and patterns (Figure I-6). The planning of a project is generally developed over several states, which typically correspond to three stages: preliminary studies, draft project, and detailed design. Road safety must be considered appropriately in each of these stages.
Preliminary studies consistency along the entire route.
definition of improvements in relation to flows.
Draft project road designs and main characteristics (for example, choice of intersection type, road widths, among others).
Detailed design safety equipment, signing and marking (señalamiento y demarcación).
treatment of critical points.
Part II Planning Stage 1 Introduction The earlier the stages of a road infrastructure project in which road safety concepts are considered and applied, the better the results obtained, both from a technical and economic point of view. Therefore, addressing the road safety of the project in the planning stage is essential, since it is in this stage where the project is in a conceptual state that allows for any modification before even capturing the idea in a blueprint.
This chapter includes some of the main conceptual elements that allow planning a road infrastructure work with road safety as a primary element.
2 Functional Classification of Roads There are various ways to classify roads so that they are recognized according to their importance, their geometric form, whether or not they belong to the road network of a certain locality, among others.
Roads and communication routes provide two fundamental services: 1) the function of providing the appropriate service for safe, constant, rapid, and efficient traffic circulation, and 2) providing access to adjacent properties.
The concept of Functional Classification of roads is based on the type of service provided by a certain road segment, which falls between the two extremes mentioned above. Six types of roads are highlighted according to their functionality: main movement, transition, distribution, collection, access, and terminal roads. This classification system encompasses two service functions: accessibility and traffic mobility.
On a main movement road, the flow of vehicles is constant and uninterrupted. On it, vehicles can travel at high speeds and it is not possible to access the road directly, but rather by means of transitions. Therefore, a road of this category has high mobility and very low accessibility.
As the user approaches their destination, they exit the main movement road and reduce their speed on a ramp that acts as a transition road. Once an appropriate speed is reached, the user enters a distribution road, which provides them with greater proximity to the destination. A collection road introduces the user to the required neighborhood. Finally, the user enters the access road, which directly transports them to the required private property, or to a terminal road where their journey ends. The result is that these latter types of roads have high accessibility and low mobility.
The different hierarchy levels of traffic roads according to their functional classification are designed to specifically fulfill a function, according to the traffic volume each one handles. Thus, depending on this figure and the population size of a certain place, the levels of service provided by the roads can be estimated. Each one has specific characteristics, which must be directed at the current and future traffic volumes that will use these roads. Among others are trip length, operating speed, trip purpose, traffic volume, access, and population.
One of the first tasks in the planning stage, therefore, consists of defining what function the road being planned will fulfill, and therefore, how it is classified according to its functionality.
Most of the technical characteristics of the road, as well as its design standards, the way it will be used by different users, its interrelation with the land use of adjacent lands, and even the design characteristics of complementary infrastructure works, will be defined based on the type of road that will be planned.
An arterial road, for example, .
2.1 Main Arterial Roads (Arterias principales) These roads provide service for long-distance transportation. They connect cities as well as link rural zones with urban zones. Therefore, they must fulfill several mobility functions, which depend on the location of the road and the land use in its surroundings. However, their main function is mobility and not access to properties.
Primary distributors (distribuidores primarios) must be clearly identified as main transport routes within a road hierarchy.
A reservation must be left on it to prevent buildings and other types of developments too close. In highly urbanized areas, this type of road will likely be of a dual carriageway standard, but as the road passes through a rural setting and the traffic flow decreases, the design standards should be reduced. This will depend on traffic flows but the emphasis should always be on maintaining through traffic. Local access traffic must be accommodated off the primary distributor (distribuidor primario), whenever possible.
The main elements to consider when planning primary distributors (distribuidores primarios) include:
Do not provide frontal accesses; Urban development permitted only set back from the road; All accesses to properties must be through local distributor roads (vías distribuidoras locales); The number of intersections must be minimized; Intersections with adequate channelization for minor flows; Where possible, pedestrians and slow-moving vehicles must be clearly segregated, Discourage parking on the road; Where necessary, parking and stopping facilities must be provided off the main road; Bus lay-bys should be provided at regular intervals; and Grade-separated intersections for extremely high flows.
If the road passes through a town where a lack of control has allowed the road to become deficient in any of the elements indicated above, there will be accident risks. If these elements cannot be reestablished by means of adequate corrective measures, then the creation of a new route must be considered, with adequate controls (that is, development control and road hierarchy standards) that are met from its inception to prevent the recurrence of accidents.
2.2 Secondary Distributors (Minor Arterial Roads) (Distribuidores secundarios (arterias menores)) In the road hierarchy, these roads belong to the level immediately below Primary Distributors (Distribuidores Primarios); they are also known as District Distributors (Distribuidores de Distritos).
The requirements for moving traffic still predominate, but they also contribute to access requirements. Although they may carry a large proportion of longer-distance traffic, they do so only as one of the main distributors to and from the national road network.
It is generally accepted that vehicle speeds are lower (50 to 60 km/h) than on primary distributors (distribuidores primarios), and that less rigorous access controls can be permitted. However, these are still important traffic routes and segregation should be maintained where possible.
District distributors (distribuidores de distrito) should be identified as main routes in the same way as longer-distance primary routes. However, their standards can be relaxed in consideration of their reduced importance as traffic routes.
The main elements to consider when planning district distributor roads (vías distribuidoras de distrito) include:
Do not provide frontal accesses; Urban development set back from the roads; Most developments should have access by means of intersections with local distributors (distribuidores locales); In exceptional circumstances, larger-scale urban development areas may have direct access, provided a high-standard intersection is provided; All intersections will generally be at the same level; Turning traffic should be segregated from through traffic; Bicycle lanes and pedestrian routes should be separated from the roadway; Pedestrian crossing points should be clearly defined and controlled; Parking on the road must not be permitted; Bus stops and other loading and unloading areas (only permitted in exceptional circumstances) should be in separate, well-designed bays.
It is very important, if safety is to be improved, to consider the needs of pedestrians and non-motorized vehicles in the design and frequency of the intersections of these roads.
2.3 Local Distributors (Distribuidores locales) Local distributors (distribuidores locales) are the main collector roads within zones or areas. They serve to carry traffic to and from the main road network at the beginning and end of trips. These include all important connecting roads in an area, but are characterized by the absence of through traffic. Local distributors (distribuidores locales) are at the level in the hierarchy from which the need for traffic movement begins to be less important than the needs of local traffic and access.
In urban areas, these roads will directly serve residential and commercial properties. The development of the streets will adapt to the existing building pattern, and non-motorized traffic will be parallel to motorized routes on sidewalks and along slow-moving lanes. In rural areas where there are only scattered settlements, the local distributor (distribuidor local) may be the most important local connection and have a mix of residential and light industrial/agricultural traffic.
The most important function of local distributors (distribuidores locales) is access. Vehicle speeds must be kept low. Frontal accesses can be permitted, but vehicular accesses from adjacent buildings should be avoided, except in those places where high-traffic generators exist.
These roads will be within or very close to residential areas. Traffic must be aware of pedestrians, especially children. Adjacent areas along these roads can become play areas, unless necessary measures are taken so that open areas between routes do not become play areas.
The most important points to consider are:
The road is for local traffic only. Long-distance traffic must be directed to a suitable, more direct alternative road; Where possible, a road with industrial traffic should not pass through a residential area; Vehicle speeds must be kept low, so long, straight roads should be avoided; Parking is permitted but, where possible, alternative off-road parking should be provided; Non-motorized traffic is as important as motorized traffic and, if possible, separate routes should be provided; If non-motorized traffic needs to use a local distributor (distribuidor local), it must be separated from motorized traffic; Depending on traffic flows, the width of the roads can vary, to provide parking or give greater emphasis to crossings; Bus stops can be located on the roadways, but near well-defined crossings. Through traffic movements must be made difficult and inconvenient, in order to discourage them.
2.4 Access Roads (Local Roads) (Vías de acceso (vías locales)) As their name indicates, these roads are for access only and mainly for residential uses (industrial accesses should at least be made from a road with local distributor characteristics). These are the streets where people live. Therefore, the concepts of road safety, personal safety, and the environment are of vital importance. Hence, access roads (vías de acceso) should allow only residential access and the design should permit only a minimum of traffic. It is better to have many short access roads (vías de acceso), linked by a local distributor (distribuidor local), than a few long roads.
On access roads (vías de acceso), the needs of non-motorized users predominate, and children will generally play in the streets. It must be clearly indicated that vehicles are intruders in these areas and that low speeds are required.
Safety and the perception of safety on access roads (vías de acceso) depend on the frequency and severity of conflict, on the type and density of housing, and on the type of existing activities. On access roads (vías de acceso), although design standards may vary, the most important elements to consider are:
Keep vehicle flows to a minimum; Eliminate all unnecessary traffic; Keep speeds low, including by deliberately and carefully placing obstructions to create complicated alignments; Where possible, keep access roads (vías de acceso) short; Where possible, use cul-de-sacs and curved roads to discourage through traffic; Intersections should be three-arm instead of four and kept compact, to assist pedestrian movement.
Pedestrians and vehicles can share the space; The width of the roadway can be reduced to emphasize the priority of pedestrians; The entry and exit points of access streets must be clearly identifiable, which can be achieved by applying special treatments to the entrances/exits, for example: changes in the geometric layout, landscaping, building, and even gateways and signage.
recovery and free zone Parking and stopping are permitted, although these facilities should be provided within the properties or in separate garage areas; The use of low curbs will allow these streets, narrow and with low alignment standards, to be used by emergency and service vehicles, or also for occasional parking, and Emergency accesses for fire vehicles can be kept clear using diagonal closures in order to eliminate parking space, or, ensuring that other owners in the sector have access via the same road, so that they keep them clear.
2.5 Pedestrian Areas (Áreas peatonales) These are areas where motorized vehicles are completely excluded. In the broadest sense, these will include all routes where non-motorized traffic has absolute priority. This would include pedestrian walks and bicycle lanes specially built for this purpose which, generally in residential areas, form a network completely separate from those for motorized traffic.
Pedestrian routes or areas should not be planned in isolation, since motorized traffic must also be accommodated somewhere. When planning new pedestrian networks and areas, the key points to consider are:
Residential, industrial, and commercial areas should be connected by pedestrian routes that are the most direct and pleasant between destinations. The use of shade trees can encourage their use.
Any detour from a direct route must be more attractive than a less secure alternative route.
All crossings with main routes must, where possible, be grade-separated, and if this is not possible, at-grade facilities (for example refuges) must be provided to minimize problems when crossing.
Vertical detours (overpass or underpass) are much less attractive to pedestrians than at-grade facilities.
The vertical and horizontal alignments of pedestrian routes can incorporate steeper slopes and sharper curves than roads for motorized vehicles.
It is necessary to maintain open-looking spaces, especially at intersections and underpasses.
In commercial areas, priority must be given to pedestrians.
When motorized vehicles are no longer permitted, it is necessary to have adequate capacity on adjacent roads -for loading/unloading, parking, and movement-, but these facilities must always be within easy walking distance.
If there are no adequate alternatives for motorized traffic, pedestrianization must be considered at certain times of the day, that is, allowing vehicle access when pedestrian flow is low (for example very early in the morning or late at night).
Connections with bus stops, parking areas, and stations are vital, so they must be adequate.
All pedestrianized areas must provide access for emergency vehicles and garbage collection vehicles.
Pedestrians are the most vulnerable group of road users, especially children and the elderly, as they are not inside a vehicle that protects them from injuries in the event of a minor collision. The same metal body that protects the driver of a vehicle can kill a pedestrian. It is essential that the transportation system considers the needs of pedestrians; they should even be considered more than other road users, since they are much more likely to be injured or killed.
Most pedestrian movement is local in nature. It takes place on sidewalks adjacent to roads, or at their edge. In traffic management, special attention must be paid to minimizing conflicts and providing pedestrian facilities that are safe, convenient, and pleasant to use.
In developing countries, sidewalks are generally obstructed by street vendors, commercial activities, or parked cars. Measures must be taken to clear the sidewalks, through better enforcement or engineering (for example, bollards at the edges of the sidewalk to prevent them from being invaded by parked vehicles).
When pedestrian flows are intense and sidewalks narrow, their widening should be considered. When pedestrians are involved in a substantial proportion of accidents at a specific location, their needs must be carefully considered in the design and selection of corrective measures. Where possible, measures that segregate vehicles and pedestrians should be used.
The use of a small radius on curbs at intersections to reduce the speeds of turning traffic can also help pedestrians cross side roads more easily at corners. A good measure is to extend the sidewalk of the main road, crossing the secondary (or side) roads and creating raised crossings. This allows pedestrians to continue at the same level as the sidewalk, while drivers have to maneuver slowly over the raised sidewalk.
To encourage their use, sidewalks must be reasonably level and well-drained. The existence of vegetation should not be allowed, as this can obstruct the sidewalk. When work is under construction, temporary alternative walkways must be provided for pedestrians displaced from the sidewalk being modified.
2.6 Roads for Cyclists (Vías para ciclistas) Bicycles and other slow-moving vehicles must be considered separately in the road system, due to their different movement characteristics, the fact that drivers tend to perceive them less than other vehicles, and that they are more vulnerable to injuries in the event of an accident. Roundabouts in particular have a poor accident record with these vehicles, but difficulties can occur at any type of intersection.
At priority intersections, slower vehicles can be protected by segregating their movements by means of channelization or providing central refuges that allow them to move in two stages.
At traffic lights, a useful method to protect slow-moving vehicles is to allow a separate phase (managed by separate signal heads, incorporating, for example, a bicycle symbol) or to give them an advantage at the start by means of a separate stop line located several meters ahead of the stop line for the rest of the traffic.
Problems at roundabouts are much more difficult and the best solution is to direct slow-moving vehicles to an alternative route. Bicycles and other small vehicles could also be allowed to share pedestrian facilities, where these exist, as a convenient alternative. In the United Kingdom, this alternative has proven to be a safe measure with flows of up to 300 per hour.
If a suitable alternative route cannot be found to avoid the roundabout, or if the number of slow-moving vehicles is significant, it might be necessary to consider a different type of intersection control.
Facilities for cyclists must be attractive, or they will not be used. They must not have large detours, or very steep slopes, or steps or dismounts, and must be well maintained.
It is essential to prohibit on-road vehicle parking, since slower vehicles will be forced to make sudden movements in their travel to dodge parked vehicles.
In new projects, potential problems can be avoided with an appropriate design of segregated alternative routes.
3. Planning for Different Land Uses (Planificación para diferentes usos del suelo) 3.1 Residential Areas (Áreas residenciales) Residential roads are the primary setting where vehicles and pedestrians interact, and where the movement function plays an increasingly smaller role relative to the more important domestic and service activities.
In older developed areas, vehicle traffic problems have gradually increased, causing many cities to suffer from traffic safety problems. While the car is increasingly necessary to access a greater range of facilities, its movement and parking occupy most of the living space, which must fulfill other functions in these areas most of the time.
In order to provide a safe environment for vehicles and pedestrians:
Streets in residential areas more than 100 meters long must be winding and have tight curves encouraging low speeds.
Traffic that is not for access should find it impossible or highly inconvenient to use residential roads as shortcuts.
Priority must be given to pedestrians, especially near buildings and in play areas.
Direct access to dwellings should be by access roads (vías de acceso) rather than by distributor roads (vías distribuidoras).
When dwellings have vehicular access by distributor roads (vías distribuidoras), an alternative pedestrian access must be provided through segregated sidewalks to access roads (vías de acceso).
Pedestrians must be segregated where possible and crossings with vehicle routes must be convenient and safe.
Parking spaces must be ample and convenient, located away from areas where children play.
Drivers need to be made aware of the priority that pedestrians have at the entrance and throughout the entire area, through geometry, surface texture, and treatment of the entrance to the area.
Large housing developments must be subdivided to minimize vehicular traffic on internal streets.
Existing grid-type networks should be modified by means of road closures or restrictions in order to create internal or external access systems.
The reciprocal visibility of drivers and pedestrians must be sufficient to minimize the risk of accidents.
Parking of trucks at night, especially those with dangerous loads, must be discouraged.
3.2 Industrial Areas (Áreas industriales) Industrial areas are very important for the economy of most countries, so it is necessary that they have safe and efficient connections with national and international markets for their raw materials and manufactured products. In most cases, these connections are by land due to the ease and flexibility of movement they offer. However, some types and sizes of cargo can be transported more easily by train, ship, or air. In more industrialized countries, the location of industry is planned to take advantage of these cargo transfer opportunities.
Lands intended for industrial use must be clearly identified from the Development Plans, placing larger-scale developments in peripheral sectors. They must be physically separated from residential areas and, if such separation is not possible, seriously relocating these industries must be considered as a long-term objective. As a partial solution, it is possible to design measures, as part of an area traffic management scheme, to restrict the negative effects of industrial activities and to limit the movement of heavy vehicles to certain areas only.
The factors to consider for the layout and design of industrial complexes are:
Sites zoned for industrial purposes must have, where possible, direct access from the district distributor network.
Each site must have sufficient off-road parking and loading/unloading space to accommodate all operational, staff, and visitor needs within the site.
Roads and pedestrian walkways must be safe and efficient means of access for workers, visitors, and all range of foreseeable vehicles, when several different industries are grouped together.
The internal circulation system must ensure that under normal circumstances, no queues form on the network.
Networks of safe pedestrian walkways and bicycle lanes must be created between the industrial areas and the most important areas where the workers live.
3.3 Commercial Areas (Áreas de comercio) Commercial areas can vary from isolated kiosks or street vendors to large shopping centers and office complexes, occupying large tracts of land. As a consequence, transportation needs can be extremely varied. In cities of developed countries, attempts are made to avoid congestion by creating a single Central Business District (CBD) and developing commercial parks on the periphery of cities. These offer ample parking and efficient connections to public and private transport networks. In more rural areas, the scale of operation is much smaller and may be limited to certain market days, but even so, these activities require adequate provision for safety and efficiency in the safe and effective movement of traffic.
To provide a good level of access to all involved, commercial areas should be served from the local and national distributor network. It is beneficial to provide separate accesses for customers and service vehicles, or to segregate them as soon as possible. This minimizes conflicts and allows for different circulation systems.
When commercial areas consist of open-air fairs, it is essential to provide adequate space, off the main road. Off-road parking must also be provided for customers.
The most important points to consider when planning these areas are:
All commercial areas must be away from the network for through traffic; and if they are on it, service roads must be provided.
Where possible, service must be provided at the rear, separated from pedestrian accesses.
Adequate parking and loading/unloading facilities should be provided within the site of each property where possible.
Customer and visitor parking must be provided off-road, preferably a common parking lot.
Parking on the road must be discouraged, and only permitted where it does not obstruct the movement of general traffic nor create conflicts with pedestrians.
Good provision of public transport to and within these areas will effectively reduce the demand for parking; and In developing countries, when main rural roads pass through commercial centers, it will be necessary to reduce speed with physical measures, such as speed humps and raised pedestrian crossings to protect pedestrians and customers.
3.4 Recreation and Tourism Areas (Áreas de recreación y turismo) As countries develop, people have more and more time for recreational activities. This increases the demand for sports and recreation centers as well as amusement parks, in addition to facilities for spectator sporting events.
Safe access and parking facilities form an important part of success, when encouraging activities related to recreation or tourism that aim to become a necessary part of the economy.
When planning recreational facilities that are not used continuously, sharing facilities (such as parking lots) with other more general uses should be considered (for example, when stadiums are not in use, their parking lots can be used for setting up fairs or nighttime truck parking). In these cases, it is important to ensure that the uses do not coincide, or if they do, that there is adequate provision to serve all users.
The most important considerations to keep in mind are:
All recreation generators should have access, depending on their size, from a district distributor road (vía distribuidora or de distrito).
Recreation land uses must be separated from residential areas. They can, in some cases, be on their periphery, as long as recreational traffic does not filter into these housing areas.
Certain recreational uses may be acceptable within commercial or industrial areas, but great caution must be exercised; It is essential to have adequate public transportation service; Parking for all participants and spectators must be separate, within or near each facility, and sufficient to meet peak demands; Pedestrian routes between entry/parking areas and the venue itself must be free of vehicular traffic and clearly signposted; When some events need to use public roads, these must be clearly separated from general traffic (periodic closures may be justified); Service areas and facilities must be segregated from general traffic and, if possible, operate on a different schedule than public use.
Some facilities, such as parking lots, may be shared with other uses.
Part III Design Stage 1 Introduction Safety must receive special attention in the initial design stage of roads or intersections. This section of the Guide refers to the accident risks associated with these designs, for both urban and rural settings.
Generally, it will be possible in the initial design stages to develop and include low-cost safety elements, which will substantially reduce accident risks. In other circumstances, the cost of primary safety elements, such as increasing the radius of curves in mountainous terrain, may be prohibitively high. In cases like these, safety must rely on secondary elements, such as the installation of signage and barriers. Final decisions will be based on financial and economic criteria and on the available research evidence. However, whatever the decision, it is very important that safety be considered throughout the entire design and construction process.
While this section relates to the effectiveness of final designs, the design process must also include decisions and recommendations on how to safely control traffic during the construction process of the road or intersection. This must include the signage and its installation in each construction section, along with details of how the interaction between construction and traffic will be managed. In general, a good design will make driving task clear, simple and consistent. The use of these principles will cause a driver to be automatically guided through a road or a conflictive situation, such as an intersection, efficiently and safely. The driving task will be the effect of the combination of geometry, signage and pavement markings, and clear priorities presented to the driver. Even when the driver makes an error, the design must allow it to be corrected, without an accident occurring, or minimize its severity if one occurs.
Road design comprises the dimensioning of the elements that comprise them, such as vertical and horizontal curves, lane width, cross sections, distances, and intersections. The characteristics of the driver, pedestrian, vehicles, and surface serve as the basis for determining the physical dimensions of the aforementioned elements.
2 Geometric Design Parameters The importance of considering road safety in the design stage lies in the large number of accidents that have been proven to occur due to errors in calculations of curves, inclinations, and road radii, among others. The main objective of design is to provide routes that maintain flow and prevent accidents.
Road design is based on standards and norms that depend on:
The functional classification of the road.
The predicted traffic volume and the types of vehicles that will travel on it.
The topography of the terrain.
The level of service.
The design speed.
2.1 Design Speed Alignments depend on the design speed that has been selected. One of the most significant aspects regarding alignment is that it must be ensured to be consistent to avoid sudden changes and that the two alignments (vertical and horizontal) complement each other and are previously coordinated.
The speed of a vehicle on a road will vary according to the type and condition of the vehicle, the driver's characteristics, the road geometry, the presence of other road users, and speed controls.
On rural roads with low flows, the successive geometric elements presented to the driver must be coherent. This coherence is generally achieved with the concept of design speed. In most current standards, the speeds used for estimating design parameters, such as sight distance, are closely related to actual speeds. This is crucial for a safe design. Many interrelations exist to predict vehicular speeds from a known geometry. The 85th percentile speed of light vehicles, V85, is commonly used as a design basis, that is, the speed exceeded by only 15% of vehicles.
Drivers will drive at a speed that reflects the predominant geometric characteristics of the road. Rigid application of a set of design standards related to speed will not necessarily mean a safe road. For example, if the terrain allows a road to be built with substantially greater curvature than the minimum standard required for the design speed, the use of minimum standards anywhere on the road will appear to the driver as a substantial reduction of the standard, creating a potential accident concentration point at that location.
The interrelations between speed, geometry, and accidents are generally poorly understood. It is very difficult to predict the effects of the interaction between different design elements, for example, is a straight and narrow road more or less safe than a winding but wide one?
2.2 Sight or Visual Distance Sight distance is the length of roadway that the driver can observe at any given moment. The available sight distance must be such that when the driver is traveling at the design speed, there is sufficient time to perform the necessary maneuver. There are three types of distance: stopping, decision, and maneuvering.
2.3 Stopping Sight Distance It is defined as the minimum distance required by the driver to stop the vehicle without impacting an object. It is the sum of the distance traveled during the perception-reaction time and the distance that must be traveled before braking. After performing the pertinent calculations, it is recommended to travel at a speed lower than the calculated stopping distance speed unless it is necessary for the driver to react faster than normal, and said speed depends on the type of maneuver required to avoid accidents.
2.4 Decision Sight Distance It is the distance necessary for the driver to observe an unexpected obstacle or signal and decide at what speed to go and which path to follow. It also contemplates the distance the vehicle user needs to perform the necessary maneuvers specifically with efficiency and safety.
In the case of decision distances, there are also recommendations and prior calculations, but care must be taken as it depends on the type of road in question, the section concerned, and the topography of the terrain.
vertical 2.5 Passing Sight Distance It is the minimum distance required for a driver to complete overtaking one or more vehicles without colliding with any vehicle traveling in the opposing lane. It is important to know that passing sight distance calculations are made considering that only one vehicle is being overtaken, so the calculated minimum distances should not be used as adequate for multiple overtakings. There are several conditions used when calculating passing sight distance, among which are:
There is a gap between the overtaking vehicle and any other vehicle traveling in the opposing lane.
The driver uses some time from arriving at the section where overtaking is permitted until deciding to do it.
The speed of the overtaking vehicle when performing the maneuver is approximately 16 km/h more than the vehicle being overtaken.
The vehicle being overtaken travels at constant speed and slows down as the overtaking vehicle is performing the maneuver.
2.6 Friction Friction can be defined as the resistance to motion between two surfaces in contact. Its magnitude is expressed by the coefficient of friction, which fluctuates between 0 (frozen road) and a little more than 1 (best conditions). It is described by two forces: one parallel to the contact surface between the two bodies, acting in the opposite direction to their motion (friction force) and another perpendicular to said surface (normal force). The surface referred to in transportation topics is the interaction between the road and the tire.
Friction is analyzed through its longitudinal and transversal components. The longitudinal direction is the one carrying the same direction of vehicle movement, while the transversal measures the available resistance in a direction perpendicular to the vehicle's travel direction.
An important topic is the available transversal friction in a curve, which has a great impact on the speed at which one can travel. Design transversal friction values are generally smaller than the friction values achieved in reality. Generally, design transversal friction values range from 0.08 to 0.16 depending on the design speed as already mentioned. When designing, the coefficient of friction is chosen based on the need to provide a surface capable of keeping the vehicle in place despite special weather conditions, avoiding abrupt changes when traveling on a curve, and providing, in addition to safety, efficiency and comfort.
The presence of water between the running surface and the tires decreases the friction between them, hence one of the most common problems derived from the surface condition: friction deficiency causing the driver to lose control of the vehicle and it to "skid," run off the road, or collide. This condition, added to other surface problems such as potholes or the presence of dangerous objects or substances, aggravates the problem and makes fatal consequences such as road accidents more likely.
The measures normally taken are to always check the road condition, for example, there are some friction tests, and also to ensure it is free of potholes and contaminants that endanger the safety of those traveling.
3 Horizontal Alignment The horizontal alignment of a road comprises straight lines, curves, and spirals. Each of these curves has a characteristic radius that changes regularly to allow the connection between different road segments.
According to studies conducted worldwide, between 25% and 30% of accidents occur on curves, and approximately 60% of these events occur on horizontal curves with one involved vehicle running off the road. If the curve surface is wet, the number of accidents there increases significantly, and accidents occur mostly at the beginning and end of the curve.
Some of the measures that have been implemented to reduce accidents on curves are:
Lengthening the radius of curvature is generally the most proposed solution; however, it is also very costly.
Improving curve warning signage, delineation, pavement markings.
Making modifications to the shoulders and sides of the curves.
The most important characteristics of horizontal curves that must be considered in design and analysis are:
Radius of curvature.
Speed differential.
Surface conditions.
Shoulders.
Lane width.
Signage.
Sight distance.
Superelevation (Peralte).
Rollovers.
Overtaking.
3.1 Radius of Curvature When vehicles travel through a curve, the centrifugal force acts in such a way that it pushes them toward the outside of the curve; the friction of the vehicle's tires and the force produced by the superelevation (peralte) counteract the centrifugal force. As speed increases, the centrifugal force increases, to a point where friction and superelevation (peralte) are not sufficient to keep the vehicle on the road, and it skids. Some vehicles roll over under these conditions due to their center of gravity.
On rural roads, the frequency of accidents tends to increase as the radius of curvature decreases. Said frequency is not influenced solely by the curve's own characteristics such as radius, deflection angle, friction, and superelevation (peralte), but also by the road alignment characteristics prior to the curve.
It is also important to consider the set of changes in direction that create expectations in the driver regarding the alignment to come; accidents are more frequent on curves preceded by a completely straight segment than those originating from changes in direction that, so to speak, prepare the driver for the change that a significant curve will produce.
3.2 Road Width This aspect is very important in the case of heavy vehicles since the front tires follow a larger radius of curvature than the rear ones, so it is necessary to widen the lane in these curves, and this change is made based on the radius of curvature, design speed, vehicle characteristics, and expected traffic volume.
The problems most frequently found when not designing an adequate road width are vehicles skidding and collisions.
3.3 Shoulders The importance of adequate shoulder space lies in the need to have a place free of objects in the event that there is an accident-damaged or mechanically failed vehicle that can be placed in a location such that it does not interrupt traffic. Shoulder erosion is a common problem.
Shoulders must be kept free of objects, plants, and other obstacles so they can be used in any emergency by road users.
4 Vertical Alignment The vertical alignment consists of straight sections known as grades or tangents that are joined by vertical curves. The greatest impact on the design of these sections comes from the terrain topography, as the designer must adapt to said conditions or, conversely, modify the terrain.
4.1 Ascending Grades (Pendientes ascendentes) The maximum speed of a vehicle depends on its center of mass. For small passenger vehicles, the center of mass is generally quite small and can keep it on the road at constant speed. For larger, heavy vehicles, it is sometimes more difficult for them to maintain speed on an ascending grade (pendiente ascendente) and they "slide back." It has been shown that heavy vehicles decrease speed by around 7% on ascending grades (pendientes ascendentes).
Maximum grades (pendientes) have been established based on the vehicle's operating characteristics, also depending on the road. It is important to emphasize that maximum grades (pendientes) should not be used frequently, especially when grades (pendientes) are long and traffic includes a large number of trucks.
The majority of problems on ascending grades (pendientes ascendentes) are:
Speed differences between one section and another.
Collisions at the maximum grade (pendiente) or at the end of the grade (pendiente).
Inadequate overtaking maneuvers.
Errors in maximum speed calculations.
4.2 Descending Grades (Pendientes descendentes) The most important aspects to consider on descending grades (pendientes descendentes) are stopping distances and the possibility of vehicle brakes overheating.
The majority of problems on descending grades (pendientes descendentes) are:
Accidents involving a heavy vehicle that loses control.
Large speed differences between heavy and passenger vehicles.
Unexpected and inadequate running surface characteristics.
Excess speed.
Grades (Pendientes) that exceed recommended degrees.
Inadequate braking time.
4.3 Vertical Curves These curves are used so that the changes occurring between tangents happen gradually; one speaks of a minimum vertical curve length, and parabolas are used for the necessary calculations.
To design vertical curves, consideration must be given to providing the driver with an adequate stopping distance as well as comfort and to it being aesthetically acceptable.
Vertical curves are classified into: crest curves and sag curves. For both types, the respective calculations must be made, based equally on the criteria mentioned above. Additionally, each curve is characterized by having a value called K; as this value decreases, the sight distance decreases, and the curve becomes steeper. In the case of vertical curves, the sight distance must be equal to or greater than the stopping distance.
4.3 Climbing Lanes On ascending sections of roads, it is recommended to add an additional lane from the moment the grade (pendiente) causes a speed reduction of 25 km/h or more in the operation of loaded trucks. The addition of the extra lane must be considered, provided that the traffic volume and percentage of trucks justify the investment.
Although the main impact produced by climbing lanes is to increase the road capacity of the ascending road section, from a road safety point of view, climbing lanes can also produce a positive impact. On mountain roads, and particularly on those with a high volume of heavy traffic, it is very important to provide users with overtaking opportunities, since otherwise, traveling several kilometers in a caravan can cause drivers to become desperate and encourage them to perform overtaking maneuvers in sites with poor visibility, causing fatal head-on collisions.
The widening with an extra lane must be carried out on the right side of the existing roadway, for the transit of slow-moving vehicles. The continuity and alignment of the existing lanes must not be modified. Furthermore, whenever a road is widened by adding a lane for slow-moving traffic, it must be designed with the same dimensions as the existing lanes.
On road sections where climbing lanes for slow-moving traffic are built, overtaking by vehicles traveling in the opposite direction must be prohibited. Such sections are not recommended to be less than 250 m in length. Short road sections between consecutive sections that have climbing lanes must be avoided.
The transition before the additional lanes for slow-moving traffic must have a taper possessing a minimum length of 70 m. They must be extended a distance equal to the length required by slow-moving traffic vehicles to reach 85% of the road design speed.
From the moment the conditions that made the climbing lane necessary disappear, an additional climbing lane length must be added, given by the following expression:
Where:
L = Lane extension length (m).
V = Design speed (km/h).
Upon completing said climbing lane extension length, a transition taper must be provided with a minimum length of 120 m.
5 Combination of Horizontal Curves and Vertical Curves Horizontal and vertical alignments should not be considered independently. They are complementary to each other, and a deficient design can confuse the driver and lead to potentially dangerous situations. Once the road is built, it is extremely difficult and costly to correct differences in alignment. Evidence suggests that initial cost savings disappear with subsequent economic losses due to accidents and delays.
Poor alignment between vertical and horizontal layouts can cause visual effects, which contribute to causing accidents and harming the road's appearance.
A misinterpreted appearance by the driver occurs when horizontal and vertical curves of different lengths appear in the same location. For example, a driver choosing their approach speed and lateral position expecting a single crest vertical curve may be surprised by the subsequent appearance of a short horizontal curve within the vertical curve. These situations are extremely dangerous.
The presentation of misleading information to the driver can be avoided by making all points where horizontal and vertical curves change coincident. Where this is not possible and the curves cannot be completely separated, the vertical curves must be entirely within the horizontal curves or entirely outside them. Furthermore, horizontal and vertical curves must be of the same length, and the chaining of their centers must coincide.
A logical design is an intermediate arrangement between alignment, offering the maximum in terms of safety, capacity, ease and uniformity of operation, and a pleasant appearance, within the practical limits of the terrain and the area traversed.
A sharp horizontal curvature must not be introduced at or near the crest of a pronounced crest vertical curve, as, especially at night, drivers will not be able to perceive the horizontal changes in alignment.
A sharp horizontal curvature must not be introduced at or near the lowest point of a pronounced sag vertical curve, as it shortens the visual field of the road ahead.
The expense of increasing the radius of horizontal curves at the end of a steep downhill section is justified to help out-of-control vehicles. Alternative measures include escape ramps where vehicles traveling at too high a speed to turn can stop safely.
In places where snow or sand may accumulate, the design must consider which characteristics reduce the probability and amount of materials that may be deposited on the road, for example, shallow side cuts must be avoided.
At interchanges and intersections where sight distance for both roads is important, the horizontal alignment and profile must be as flat as possible. Furthermore, whenever possible, sight distances well above the minimum must be provided.
On two-lane roads, where combinations of curves are likely, straight sections with good passing sight distance must be provided to give the opportunity for safe overtaking.
6 Roadside Design 6.1 Background Roadside design, as a component of the integral design of a road, is a relatively recent concept and forms part of the general concept known as "Forgiving Highways." Most road design components were established in the late 1940s and in the 1950s. These components include horizontal alignment, vertical alignment, hydraulic design, sight distance, among others.
The basic concept of road design has been maintained for decades. Roadside safety design did not begin until the late 1960s, after much discussion, and it was in the 1970s when this type of design was incorporated into road projects worldwide. In Costa Rica, it is not until 2009 that the engineer and professor at the University of Costa Rica, Germán Valverde González, initiates the development of a technical manual for roadside analysis and design, which incorporates into this guide the state of the art on this matter, and precisely, the draft, in its current version, of the manual titled "Manual SCV: Guía para el Análisis y Diseño de Márgenes de Carreteras de Costa Rica", by engineer Valverde, is attached as an Annex. It is clarified that the document in question is still under review and has not yet been published in its final version.
6.2 The Concept of Forgiving Highways There are many reasons why a vehicle leaves the road and suffers an accident on the roadside, including:
driver fatigue or inattention excess speed driving under the influence of drugs or alcohol avoiding a crash running surface conditions such as rain, presence of loose materials like gravel, oil, among others mechanical failure of the vehicle poor visibility Regardless of the cause for which a vehicle leaves the road, a roadside free of fixed obstacles and slopes (taludes) with gentle inclinations gives the driver the opportunity to steer their vehicle back onto its travel lane. The concept of "forgiving highways" consists of allowing errant vehicles to involuntarily leave the road and encounter a roadside whose design reduces the consequences of the accident.
The concept of "forgiving highways" has been refined to the point that roadside design, in many countries, has been incorporated as an integral part of road infrastructure design criteria. The design options to reduce obstacles and other potential hazards on the roadside are, in order of preference, the following:
remove or eliminate the obstacle or hazard redesign or modify the obstacle so it can be safely traversed relocate the obstacle to a site where it is less likely to be hit reduce the severity of a potential impact using an appropriate breakaway device protect from the obstacle by means of a road restraint system (such as a safety barrier or an impact attenuator, among others) designed to contain and redirect vehicles.
delineate or mark the obstacle in case the previous options are not feasible or appropriate.
6.3 Vehicle Restraint Systems A vehicle restraint system is a device whose purpose is to reduce the severity of the consequences of run-off-road accidents, for both the vehicle's occupants and other road users and third parties located nearby. Its function is to substitute the collision of the vehicle against the obstacle with a more controlled impact against the system itself. Therefore, its function is not to prevent run-off-road accidents, but rather to reduce their severity.
Classification of systems according to their function and location:
Safety barrier: is a longitudinal system parallel to the vehicular flow; its purpose is to contain and redirect vehicles that lose control and leave the road.
Special safety barriers:
Aesthetic value systems: used in national parks and protected areas to enhance the beauty of the landscape.
Motorcyclist restraint systems: are safety systems designed to prevent motorcyclists from suffering serious injuries if they lose control and collide with the safety barrier.
Cyclist and pedestrian systems: used on pedestrian bridges or cycle paths.
Barrier stiffness is defined as the capacity it possesses to withstand stresses without acquiring large deformations or displacements. Rigid systems hardly deform if impacted by a vehicle, therefore resulting in more aggressive elements for road users.
Impact attenuator: is a device that stops a vehicle at a deceleration rate tolerable for its occupants or redirects the vehicle away from the potentially dangerous object.
Bridge parapet: is a system analogous to a safety barrier, but is designed specifically for edges of bridge decks, bridges, crests of retaining walls, and similar works Barrier terminal: is a system designed to reduce the probability that a vehicle will be launched, roll over, or suffer excessive deceleration if it impacts the end of a safety barrier. It generally includes the anchorage of the safety barrier.
Transition: a section of barrier whose stiffness gradually increases to join a flexible or semi-rigid system to a rigid system or a fixed object.
Arrestor beds, escape ramps, or braking ramps: are areas adjacent to the roadway where heavy vehicles can stop if they lose control. The terrain is formed of clean materials, difficult to compact, and with a high rolling resistance coefficient. Arrestor beds can also be a paved escape lane.
6.4 The SCV Manual The "SCV Manual: Guide for the Analysis and Design of Costa Rican Roadside Margins" compiles the design criteria governing safety barriers, barrier terminals, and transitions.
The criteria established in this guide are applicable to new construction projects or conservation, rehabilitation, and reconstruction projects for roads that include intervention in the road margins.
This guide establishes as a reference for design the containment levels defined by the EN 1317 standard, and in the event that the system to be installed as a solution corresponds to a safety barrier tested under the NCHRP Report 350 standard, the SCV Manual defines the equivalences between the test levels of both testing standards.
The analysis and design procedure of the SCV Manual is aimed at determining whether the margin of a road in a given section presents conditions such that the consequences of a run-off-road accident are not severe. For this condition to be met, the margin must be free of potentially hazardous obstacles and the terrain must have a relatively flat cross-section.
The first stage consists of determining whether there is a clear zone free of potential hazards in the road margin, for which it is necessary to:
* Conduct an inventory of the elements located in the road margins, measure their dimensions and location with respect to the travel lanes of the road, and establish which could be potentially hazardous (for road users or third parties) due to their dimensions and characteristics.
* Measure widths and slopes of the terrain in the road margin.
* Establish the available clear zone (zona libre).
* Calculate the necessary clear zone (zona libre) for each section of the road.
If the available clear zone (zona libre) is greater than or equal to the necessary clear zone (zona libre), the road margin is considered safe and it is not necessary to implement any measure.
If it is necessary to install a vehicle restraint system (sistema de contención vehicular), the analysis and design procedure allows establishing the necessary containment level of the system, as well as designing the system, that is, establishing the system's layout parameters (length, flare, transverse layout, etc.).
Part IV Construction Stage 1 Introduction Road infrastructure projects aim to promote population mobility through adequate roads that the engineer and design planners must build with sufficient knowledge to define the necessary structure, materials, and planning.
Road construction is one of the engineering works that can present the most difficulties since the terrain conditions, the target population, the direct and indirect consequences, among other factors, vary not only from one project to another but from one square meter to another on the same road. This is why a highly detailed plan for the works involving the construction of these projects must be prepared and coordinated.
When carrying out maintenance construction on existing roads, many elements are involved in which user safety may be implicated. The driver who normally travels on a road comes to know it so well that they become accustomed to it and do not expect to encounter elements foreign to it.
In the task of driving, the driver dedicates themselves to performing a series of tasks that allow them to fulfill their objective of traveling. Firstly, they must focus on operating the vehicle they are traveling in, properly coordinating all the devices it possesses, in such a way as to allow the correct operation of the vehicle. In addition to this, they must take into account the road conditions and the environment surrounding the vehicle, as well as the navigation route they must follow to achieve their objective.
Another element is the potential distractions present inside the vehicle cabin, attention to passengers, incoming telephone calls, among others.
When the previous elements are combined, a level of complexity is established that is not perceived by the driver when performing the driving task, as they perform it automatically according to their experience. However, if, despite the mentioned elements, the factor of the existence of unusual elements on the road is added, an accident may occur.
It is, therefore, that when carrying out construction on existing roads, it must be a priority in the responsibilities of the project manager to provide guidance to the user's driving, through adequate signage that indicates how to drive through the area where works are being carried out.
Even so, after the driver is instructed on how to perform their task in the work zone, there must be a safety provision in case some user does not perceive the change in the road conditions they are traveling on.
The elements involved in road safety during road construction stages, as well as signage and the continuity of vehicular flows, must be designed prior to the execution of the work. The personnel involved as part of the signage and guidance for drivers must possess the necessary training to perform their task optimally.
2 The Construction Process of a Road 2.1 Generalities The roadway (calzada) is the surface built on soil or on a structure for the movement of people and vehicles. The objective of the roadway (calzada) is to make circulation from one point to another possible, providing vehicles with a comfortable and safe surface.
Building a roadway (calzada) involves using standards that meet needs and provide numerical solutions for the respective design. It is assumed that those solutions correspond to loads and effects that are known to produce an impact on the surface.
2.2 Fundamental Characteristics of Roads There are fundamental characteristics that a road must have, such as:
* It must be built in the most economical and efficient manner.
* It must be preserved for the longest possible time without needing repairs.
* Driver safety is guaranteed if they can brake at any moment while maintaining adhesion between the tires and the roadway (calzada) and if the signs along the route guide them safely, efficiently, and along the correct path.
* User comfort is linked to the absence of imperfections that can cause changes of different kinds during the journey.
* It must deform very slowly under the effect of loads.
* The geometric layout must comply with established parameters and standards.
* The characteristics of the road must not be separated from the characteristics of the vehicles.
3 Traffic Control at Work Sites 3.1 Introduction In areas where construction or maintenance work is carried out, traffic regulation is an essential part of the works to be performed.
Temporary traffic control on roads is very important when their normal operation is suspended, as it provides continuity of movement for vehicles, pedestrians, and other system users, as well as traffic operation and access to properties.
The primary function of temporary traffic control is to provide safety and effective movement for road users through or around areas where maintenance or construction work is being carried out. Equally important is the safety of the workers operating at the site. Due to the constant changes in the conditions of the zones where works are carried out, and because these changes are unexpected by road users, there is a high degree of vulnerability for the workers.
Improvements in the performance of road users must be achieved through programs that cover the nature of the works, the duration time of their execution, anticipated effects on system users, and possible alternate routes and modes of travel. Such programs have resulted in a significant reduction in the number of users traveling through construction work zones, with a consequent potential reduction in the number of conflicts.
Operational improvements can be achieved through the use of Intelligent Transportation Systems in work zones. The use of this technology, such as portable cameras, highway advisory radio, variable speed limits, ramps, travel information, among others, is associated with an increasing trend towards safety for both workers and users, as well as ensuring more efficient traffic flow. The application of Intelligent Transportation Systems in work zones has demonstrated its effectiveness in providing traffic monitoring, as well as management of collected data and travel information.
Temporary traffic control plans and devices must be the responsibility of authorities from a public body that has jurisdiction in guiding users of the road system. There must be adequate regulations or statutes for the implementation of the necessary regulations for users, parking controls, speed zoning, and traffic incident management. Such statutes must provide sufficient flexibility in the application of temporary traffic control to be responsive to the changing conditions in work zones.
Planning for temporary traffic control must begin in the planning stage and continue through the design, construction, and operation stages.
Although it is assumed that users must exercise caution when using the system, special care is necessary in the application of temporary traffic control techniques. During road work activities where temporary traffic control is applied, it is recommended that commercial vehicles follow different routes than passenger vehicles, due to weight, visibility, or geometric restrictions. Vehicles carrying hazardous materials must also follow different routes than other vehicles.
The safety of road users and workers, and accessibility in temporary traffic control zones must be an integral and priority element in every project from planning through design and construction. Similarly, maintenance work must be planned and carried out in a way that considers the safety and accessibility for all motorcyclists, cyclists, pedestrians (including those with disabilities), and workers. If the temporary traffic control zone includes a grade separation, coordination must take place with the corresponding railway company or transit agency.
3.2 Temporary Traffic Control Zones A temporary traffic control zone is an area on the highway where conditions for road users have been modified due to a work zone, an incident area, or a special event, demarcated by the use of temporary traffic control devices, uniformed officers, or authorized personnel.
A work zone is an area on the highway where there is construction, maintenance work, or general work activities. It is typically demarcated by signs, channelizing devices, barriers, pavement markings, or work vehicles. It extends from the first warning sign, or strobe, oscillating, rotating, or high-intensity lights on a work vehicle, to the end-of-road-work sign or the last temporary traffic control device.
An incident area is an area on the highway where temporary traffic controls are imposed by authorized officers in response to a traffic incident. It extends from the first warning device to the last temporary traffic control device or the point where road users return to the original alignment line.
A previously planned special event often creates the need to establish altered traffic patterns to manage the increased traffic volume generated by the event. The size of the temporary traffic control zone associated with a special event can be small, or it can extend to closing a street for a festival, or even across an entire municipality for even larger events. The duration of the temporary traffic control zone is determined by the duration of the event itself.
A temporary traffic control zone is divided into four sections:
* Advance Warning Area: section of the highway where the road user is informed about the upcoming work zone or incident area.
* Transition Area: section of the highway where users are redirected out of their normal path. These areas usually involve strategic uses of cones, portable signs, strobe, oscillating, rotating, or high-intensity lights, or arrow boards, as channelizing devices, because stationary channelization is impractical for redirecting the normal path.
* Activity Area: this is the zone on the highway where work activities take place. It comprises the workspace, the traffic space, and the spacing necessary to separate and avoid accidents between traffic and workers.
* Termination Area: section of the highway where road users return to their normal driving path or route. It extends from the end of the work area to the last temporary traffic control device indicating the end of the road work zone.
3.3 Traffic Management Plans A temporary traffic control plan describes measures for temporary traffic control intended to be used to safely direct road users through work zones or an incident area. Traffic management plans play a vital role in providing continuity to the effective flow of road users when a work zone, incident, or other event temporarily disrupts the normal flow. Important auxiliary provisions that are not conveniently specified in the project plans can be incorporated as Special Provisions within the temporary traffic control plans.
Temporary traffic control plans range from being very detailed to a simple reference to typical standardized illustrations from manuals, or specific drawings contained in the contract documents. The degree of detail depends entirely on the nature and complexity of the situation.
Temporary traffic control plans must be prepared by persons knowledgeable about the fundamental principles of temporary traffic control and the work activities to be performed. The design, selection, and location of devices for a temporary traffic control plan must be based on technical criteria. Coordination must occur between adjacent or overlapping projects to verify the compatibility of traffic control between them.
Traffic control plans must be developed for all road construction, maintenance operations, and incident management, including minor maintenance and general works intended to occupy the temporary traffic control zone. Likewise, planning must be considered for all types of users.
Necessary provisions must be incorporated into the temporary traffic control process to ensure the effective continuity of the pedestrian circulation surface, as well as to ensure it is accessible to persons with disabilities. In areas where existing pedestrian routes have been blocked or destroyed, information about alternate routes must be provided for use by pedestrians with disabilities, particularly those with visual impairments.
Access to temporary bus stops, crossings at intersections with accessible pedestrian signals, and other route elements must be considered in the temporary locations enabled for pedestrians. Additionally, channelizing devices and barriers must be provided for persons with visual impairments.
Provisions for the effective continuity of transit service must be incorporated into the temporary traffic control planning process, since public service buses often cannot be diverted in the same manner as other vehicles (particularly for short-duration projects). Provisions must also be included for the effective continuity of railway service and acceptable access for owners adjacent to the work zones, both property owners and business owners.
Reduced speed limits must be used only in specific portions of the temporary traffic control zone where restrictive conditions and features exist. However, frequent changes in speed limits must be avoided. A temporary traffic control plan must be designed in such a way that vehicles can travel through the work zone with a speed limit restriction of no more than 10 mph (about 16 km/h). A greater reduction to said value in the speed limit may be used only when required by the restrictions in the temporary traffic control zones. In areas where this speed reduction is justified, additional safety devices must be provided for drivers.
Speed reduction zoning must be avoided because drivers will only reduce their speeds if they clearly perceive the need to do so. Research has shown that a large reduction in the speed limit, around 30 mph (about 50 km/h), significantly increases the potential for traffic crashes. Smaller reductions in the limit, somewhat greater than 16 km/h, cause small changes in speed variation and decrease the potential for accidents.
3.4 Temporary Traffic Control Devices Traffic control devices are defined as all those signs, markings, signals, and other devices used to regulate, warn, or guide road users, placed on or adjacent to a street, road, private way, pedestrian path, or bikeway by an authority of a public body or officers with jurisdiction on the matter. Signs in the temporary traffic control zone communicate general and specific messages by means of words, symbols, and arrows, and have the same three categories as the types of signs for different road users: regulation, warning, and guidance.
The design and application of temporary traffic control devices used in road work zones must consider the needs of all road users, whether motorcyclists, cyclists, pedestrians, or drivers, including those with disabilities.
Regarding the colors to be used, the Manual Centroamericano de Dispositivos Uniformes para el Control del Tránsito must be used.
All signs used at night must be retroreflective, with reflective or illuminated surfaces, so that drivers can identify them in all types of conditions. They must be made of rigid materials and have internal or external illumination.
The minimum height, measured vertically from the bottom of the sign to the pavement surface, for signs installed on rural roads, must be 1.5 m.
Adequate maintenance must be provided to temporary traffic devices, regarding clarity, visibility, and correct position. If a device has significantly lost its legibility, it must be replaced.
If a temporary traffic control zone requires regulation different from the existing one on the road, the permanent regulation devices must be removed or covered, and substituted with devices of appropriate temporary regulation.
3.5 Control Techniques It is desirable that the design of the temporary traffic control zone be dimensioned and signed in such a way that users can travel through it agilely and safely without the need for personnel guidance or control; however, in some situations this is not possible, and it becomes necessary to apply manual control techniques.
The main function of the hand-signaling control technique is to ensure that the movement of both vehicles and pedestrians through the temporary traffic control zones is carried out safely, and at the same time is efficient and provides protection to workers and equipment used in construction.
Flaggers (abanderados) who perform control using hand signals must possess the necessary training to do the job, since they are responsible for providing the necessary road safety for both workers and users. They must have the appropriate high-visibility clothing, with retroreflective materials of colors appropriate for their day or night function.
Primarily, hand-signal devices with "STOP" or "SLOW" indications must be used. The use of flags must be limited only to emergency situations, or to sites where both traffic volume and speed are low.
The points where flagging (abanderamiento) is used must be located with sufficient anticipation distance from the work areas, such that traffic has sufficient stopping distance before entering the workspace.
Part V Operation Stage 1 Introduction Every road infrastructure project that is carried out, from its initial planning and design stages, must be shaped to comply with certain specifications that categorize it as a road that provides the necessary road safety for each of the users who travel on it, whether pedestrians or drivers. Additionally, in the construction stage, the necessary guidelines must be provided so that users perceive continuity in the service, such that it does not affect their daily driving task.
The parameters and indications contained in this manual must be reviewed and included in each of the project stages. By doing so, it will be ensuring that the conditions under which the road will operate are optimal.
Despite the above, even if the road's functioning is such that the user is provided with the necessary road safety to use the system optimally, and they have no setbacks regarding their safety, there are random factors that cannot be taken into account when carrying out the project construction, and which can lead to fatal errors involving potential accidents.
This chapter examines the inspections that must be carried out on the road after it is put into operation, as well as the follow-up that must be given to them. On the other hand, certain guidelines to follow regarding traffic accident management plans are mentioned.
2 Road Safety Inspections Road safety inspections refer to the review of roads in the field after they enter service. The objective to be fulfilled with road safety inspections corresponds to the identification of hazardous aspects, deficiencies, or shortcomings that the road possesses, which are susceptible to triggering an accident.
A primary objective of road safety inspections is the detection of safety problems at points that represent potential danger to road users, so that the correct measures can be adopted to eliminate those problems.
The following aspects are usually inspected as part of road safety inspections: road functionality, layout, cross-section, intersections, signage, lighting, margins, environment, and passive safety elements.
After inspecting these elements, the process continues with the preparation of preliminary office work, where traffic data, accident rates, available statistics, and road characteristics are analyzed.
Next, field work is carried out, where the data computed so far is verified, day and night inspections are performed, traveling the roads at different speeds, and intersections, connections, interchanges, and other points of interest are analyzed independently.
In the next phase of the process, the potential accident scenarios must be identified, as well as preparing a list of the safety problems that were identified, with their respective recommendations for providing a solution.
Priorities must be established, after analyzing the risks presented by each problem, and with this, prepare an inspection report, which describes each of the problems found in terms of the accident risk they involve, and their respective recommendations or measures to implement to resolve them.
It is important, upon finishing the road safety inspection, that the proposed measures are executed, and even more so, that they are given adequate follow-up over time, to monitor whether they are fulfilling the function for which they were implemented.
3 Traffic Accident Management Plans Traffic accidents represent one of the major causes of death worldwide, generating an enormous economic and social burden. This is why their impact must be minimized with prevention measures or practical guides for the management of the patients involved, whether at the event occurrence site or at health care sites.
At the time of a traffic accident, authorities must organize themselves to have comprehensive traffic management plans that, in the most efficient way possible, resolve the conflicts generated by the accident.
The action radii of the medical centers near the road in question must be considered, and with this, the optimal routes for patient transfer at the time of an incident must be determined.
The emergency vehicles used in the task of assisting those involved in traffic accidents, such as ambulances, police patrols, fire trucks, among others, must have accurate, first-hand information about these routes, leading to minimizing arrival times from their origin to the accident site.
An organization of the network aimed at minimizing the effects of traffic accidents, contemplating the health care sites near the road in question, will significantly mitigate the economic and social impact caused by them.
Summary In Costa Rica, a guide is still not available to guide designers and builders in the design of road margins, as well as in the selection and placement of vehicle restraint systems (sistemas de contención vehicular), which include safety barriers, bridge parapets, crash cushions, barrier terminals, and transitions between systems.
Currently, the regulation in force is the provision MN-06-2006 "Flexible Beam Type Steel Barrier (Barrera de Acero Tipo Viga Flexible)", which regulates the selection of materials and installation of guardrails in our country. This focuses on standardizing the technical aspects for the supply and installation of flexible beam type steel barriers, leaving out other types of vehicle restraint systems (sistemas de contención vehicular) available on the market.
A review of the instructional document allowed verifying that it is a summary of some criteria set forth in other foreign manuals and does not reference the evaluation and treatment of obstacles in the road margins, the dimensioning of the clear zone (zona libre), and the criteria for selecting the system type as well as the barrier's containment level and deflection, based on important parameters of the road, vehicular flow, and the severity of a potential accident.
This article presents the analysis and adaptation of international criteria proposed for the road safety design of Costa Rican margins, which are included in the first version of the "SCV Manual: Guide for the Analysis and Road Safety Design of Costa Rican Road Margins". This manual, prepared by the author, aims to be an official practical guide for the identification of obstacles, the treatment of hazards, and the selection and layout of road restraint systems; one that adjusts to the safety needs currently present on Costa Rican roads.
1. Background Events known as "run-off-road crashes"—those traffic accidents that occur when a vehicle leaves the roadway travel surface and collides with a fixed object, overturns, or strikes a third party on the roadside margin—cause more than 30% of highway fatalities.
For this reason, improving and conditioning the margins of the road network is a highly effective measure in reducing traffic accident victims. To perform this work adequately, it is necessary to have substantiated and uniform technical criteria, and thus arose the author's initiative to develop a technical manual for roadside margin analysis and the placement of vehicle restraint systems for Costa Rica.
The design of highway margins, as a component of the comprehensive design of a highway, is a relatively recent concept, and is part of the general concept known as "Forgiving Highways." Most of the design components of a road were established in the late 1940s and in the 1950s. These components include horizontal alignment, vertical alignment, hydraulic design, sight distance, among others.
Roadside safety design of highway margins did not begin until the late 1960s, after much discussion, and it was in the 1970s when this type of design was incorporated into highway projects worldwide.
In Costa Rica, it was not until 2007 that the engineer and professor at the Universidad de Costa Rica, Germán Valverde González, began the preparation of a technical manual for the analysis and design of highway margins, which incorporates the state of the art on this subject into this guide.
The preparation of the guide began in 2007 with the proposal for the Final Graduation Project entitled "Review of criteria for the placement of vehicle restraint systems in Costa Rica," by then-student Eng. Ruth Quesada, a research project conducted under the direction of Eng. Valverde (Quesada, R., 2008).
Subsequently, between 2008 and 2009, a validation study of the criteria and recommendations set forth by Quesada (2009) was carried out, which consisted of analyzing the safety conditions existing on the margins of the Florencio del Castillo Highway, in light of the technical criteria proposed by Eng. Valverde (Valverde, G., 2009). This work was funded by the Laboratorio Nacional de Materiales y Modelos Estructurales (Lanamme) of the Universidad de Costa Rica with funds generated by I-CISEV. Officials from the Dirección General de Ingeniería de Tránsito (DGIT) and the Consejo Nacional de Vialidad (CONAVI) of Costa Rica participated in the execution of this work.
To finalize the preparation of the manual, in July 2010 Eng. Valverde registered the research project called "Infrastructure and Road Safety" with the Vicerrectoría de Investigación of the Universidad de Costa Rica, and thus the first version of the "SCV Manual: Guide for the roadside safety analysis and design of highway margins in Costa Rica" (Valverde, G., 2010) – SCV Manual from now on – was prepared.
To prepare this document, a review of various international technical manuals was conducted, from which those methodologies best suited to the conditions of Costa Rica, or those considered to have a higher level of technical development, were adopted. As part of the research, the adopted methodologies and criteria were adapted.
Thus, for example, the criteria for determining whether the cross-section of a ditch (cuneta) is considered traversable are an adaptation for Costa Rica of the criteria recommended in AASHTO (2002); the criteria for establishing the containment level of safety barriers are a combination of methodologies from Spain, Italy, and Germany, with an adaptation to the traffic and speed conditions of national roads; while the design methodology for escape ramps has been adapted from the Mexican standard SCT (2007).
2. The Concept of Forgiving Highways There are many reasons why a vehicle leaves the road and suffers a crash on the roadside margin, including: driver fatigue or inattention, excessive speed, avoiding an obstacle on the road, presence of water on the highway, mechanical failure of the vehicle, poor visibility, among others.
Regardless of the cause for which a vehicle leaves the road, if the roadside margin is free of fixed obstacles and has slopes (taludes) with gentle inclinations, the driver will have the opportunity to redirect their vehicle back to their travel lane without colliding and without suffering damage.
The concept of "Forgiving Highways" consists of allowing errant vehicles to involuntarily leave the road and encounter a roadside margin whose design reduces the consequences of the crash.
3. Run-off-Road Crashes Run-off-road crashes comprise those eventual events in which an errant vehicle leaves the roadway and collides with a fixed object, encroaches onto another road, or descends a steep slope (talud) and overturns, potentially causing harm to the vehicle occupants or third parties.
The run-off-road event can be produced by direct causes related to the road infrastructure, the environment, the driver, or the vehicle. A run-off-road crash can also occur indirectly, as a secondary effect of another event, for example, a vehicle could leave the road after rear-ending another vehicle.
Strategies to reduce run-off-road crashes focus on one of the following objectives:
. Prevent vehicles from leaving the road.
. Minimize the probability that a vehicle collides with a dangerous fixed object or overturns if it descends a steep slope (talud).
. Reduce the severity of the crash by installing safety devices.
Ideally, road design should be oriented toward keeping the vehicle in the travel lane and providing safe zones at the margin of the travel lanes, where the driver can stop or reduce speed and regain control without intercepting objects or vulnerable third parties.
If the zone at the roadside margin is a flat, compacted area free of fixed objects, the probability of a crash occurring is minimized, since in most cases the driver is able to stop the vehicle or redirect it safely.
If an obstacle or hazard cannot be eliminated, relocated, or modified for technical, economic, or environmental reasons, vehicle restraint systems must be placed to reduce the severity of the crash. Roadside equipment such as breakaway or frangible posts and bases, safety barriers and their terminals, bridge parapets (pretiles de puentes), and impact attenuators (atenuadores de impacto) are elements intended to reduce the severity of injuries to vehicle occupants and material losses resulting from the collision; therefore, they should be used only if it is not possible to implement any other treatment, since vehicle occupants are not exempt from suffering some type of injury or material loss if they collide against the system.
4. Vehicle Restraint Systems When there is no reasonable, technical, or economic possibility of resolving risk situations through a design intervention; the elimination, displacement, or modification of the obstacle; or the expansion of the space between the outer edge of the road and the hazard object; all required safety devices must be planned, through specific regulations and recommendations, to achieve the objective of preventing and protecting users from potential hazards.
Vehicle restraint systems are devices installed on highway margins, with the purpose of retaining and redirecting vehicles that leave the road out of control, reducing damage and injuries, both for the occupants and for other road users and people or objects located nearby.
Collision with a vehicle restraint system constitutes a substitute crash for the one that would occur if this mechanism did not exist, and one with more predictable and less severe consequences; but this does not mean that vehicle occupants are free from risks.
The SCV Manual compiles the design criteria governing safety barriers, barrier terminals, and transitions. It should be highlighted that the term design is used here as a synonym for the selection of the system's containment level and its placement with respect to the obstacle and the highway.
The criteria established in this guide are applicable to new construction projects or conservation, rehabilitation, and reconstruction projects of highways that include intervention of the highway margins.
5. Crash Tests and the SCV Manual 5.1 Generalities Crash tests are standardized tests that have been designed to evaluate one or more of the main factors affecting the performance of vehicle restraint systems, such as structural behavior, risk to vehicle occupants, and the post-impact behavior of the test vehicle. Their purpose is to verify the adequate functioning of the system, to guarantee the safety of vehicle occupants, other road users, and vulnerable third parties.
There are two standards for the evaluation of vehicle restraint systems, the European standard EN 1317 and the American standard NCHRP Report 350. These standards contain standard procedures for testing, evaluation, and classification of vehicle restraint systems.
The test standards define the following parameters to evaluate the performance of vehicle restraint barriers and define the acceptance limits as well as the technical classes:
. Containment level.
. Impact severity.
. Deformation of the restraint system.
. Redirection capacity or vehicle trajectory after impacting the system.
The containment level is the transverse kinetic energy that a system is capable of retaining in a controlled manner, without the vehicle passing through the system or overturning. No relevant part of the system must detach or penetrate the vehicle's passenger compartment during the test, so that the vehicle remains stable during and after impact with the device. Slight pitching, rolling, or inclination may be acceptable.
Impact severity is defined as the risk level of suffering injuries for the vehicle occupants as a consequence of a collision. A system that is capable of containing a truck is useless if, when containing a light vehicle, it causes serious injuries or the death of its occupants; this is why parameters have been developed worldwide that allow quantifying the severity of the impact, among which the decelerations measured inside the vehicle and the deformation of the passenger compartment stand out.
The deformation of the system is described by two transverse distances measured during full-scale impact tests: working width (W) and dynamic deformation (D). They represent the maximum transverse space that, under the standardized impact conditions of the test, has been used by the system during its deformation. If the deformation of the system is greater than the transverse space between it and the hazardous zone, then the system does not truly protect the user from the hazard (See Figure 1).
The working width (W) is the distance between the face closest to traffic before the impact, and the farthest lateral position that any essential part of the system or vehicle reaches during the impact.
The dynamic deflection (D) is the maximum dynamic lateral displacement of the system face closest to traffic.
The dynamic deflection and the working width allow the location of the safety barrier with respect to the obstacle or hazardous zone to be fixed.
The exit angle is a parameter used to measure the capacity of the safety barrier to provide the impacting vehicle with an exit direction as parallel as possible to the roadway axis.
Excessive lateral deformation of the system can produce "pocketing," which generates a greater exit angle than the entry angle; as a consequence, the vehicle can impact other vehicles traveling on the same road or can even impact the barrier on the opposite side again.
5.2 Comparison of the Test Standards and Criteria Adopted by the SCV Manual There are important differences between the test characteristics of the European standard EN 1317 and the American standard NCHRP Report 350, which have been analyzed by Valverde (2010) and considered in the SCV Manual to assess the use in Costa Rica of restraint systems tested under the criteria of both standards.
Containment Level The test criteria for safety barriers defined by the European standard EN 1317 are summarized in Table 1, and Table 2 shows the classification this standard makes of safety barriers according to the results of said tests.
The containment levels established by the American standard NCHRP Report 350 for vehicle restraint systems are summarized in Table 3. Note that the tests differ with respect to those specified in the EN 1317 standard.
The test criteria of the EN 1317 and NCHRP Report 350 standards can be compared according to the containment level or transverse kinetic energy that the system is capable of retaining in a controlled manner.
The European standard EN1317 establishes 6 test levels or technical classes (See Table II-3). In Table II-11, the transverse kinetic energy values corresponding to the tests to which safety barriers of each containment level are subjected are shown. For each of the tests, the type of vehicle and its characteristics (mass and center of gravity height CG), the speed and impact angle of the vehicle, and the transverse kinetic energy the system is capable of retaining are shown.
In the United States of America, 6 test levels or technical classes have been defined, which are defined in the NCHRP Report 350.
Table 4 shows, for each of the technical classes, two of the types of vehicles used in the full-scale test (for comparison purposes with the test levels conducted under the EN 1317 Standard) and their characteristics (mass and center of gravity height CG), the speed and impact angle of the vehicle, and the transverse kinetic energy the system is capable of retaining.
For purposes of comparing the containment levels defined in both standards and attempting to equate them, the maximum incident transverse kinetic energy to which safety barriers are subjected is used.
The graph in Figure 2 shows the maximum incident transverse kinetic energy values corresponding to the different containment levels established by both standards. This figure allows observing the comparison of the test levels according to the transverse kinetic energy that the system is capable of absorbing during the test.
For purposes of comparing the containment levels defined in both standards and attempting to equate them, the maximum incident transverse kinetic energy to which safety barriers are subjected is used.
The graph in Figure 2 shows the maximum incident transverse kinetic energy values corresponding to the different containment levels established by both standards. This figure allows observing the comparison of the test levels according to the transverse kinetic energy that the system is capable of absorbing during the test.
It is important to clarify that only the incident kinetic energy is being compared; therefore, two equivalent systems (that are capable of absorbing a similar amount of kinetic energy), for example TL3 and TL4, are not necessarily capable of retaining the same vehicle, since the tests are conducted with different types of trucks. Thus, a taller vehicle can lean over a barrier and overturn even though the system has the capacity to dissipate the kinetic energy of the impact. It is worth noting that in the United States, a pickup-type vehicle is used to evaluate low-containment barriers, whereas in Europe a smaller car is used.
Classes TL5 and TL6 absorb up to 76% more transverse kinetic energy during the impact than the immediate lower classes TL3 and TL4, implying that the increase in the containment level is not gradual. Therefore, there is a range of energies not included in the American standard, which correspond to classes H2 and H3 of the European standard.
In turn, in the graph of Figure II-8 it is clearly observed how the containment levels established in the European standard increase in a more gradual manner. Furthermore, it is observed that the NCHRP Report 350 standard does not include a test level that is equivalent (in terms of maximum incident transverse kinetic energy) to the H4b containment level of the EN 1317 standard.
In terms of the maximum incident transverse kinetic energy, the test levels established by the EN 1317 standard present a more gradual increase than those standardized in the NCHRP Report 350, which allows selecting containment levels that better adjust to different particular conditions at different sites.
Therefore, in this guide, the containment levels defined by the EN 1317 standard are established as a reference for design. In the event that the system to be installed as a solution corresponds to a safety barrier tested under the NCHRP Report 350 standard, this guide defines the equivalences between test levels shown in Table 5.
These equivalences between test levels only consider the maximum transverse kinetic energy that the safety barrier is capable of absorbing during the impact.
Systems considered equivalent do not behave in exactly the same manner, and in each case, other factors such as working width, dynamic deformation, and severity level must be analyzed to select the most appropriate system according to the site conditions.
Severity Levels Under the EN 1317 standard, the tests of safety barriers with containment levels T3, N2, H1, H2, H3, H4a, and H4b, in addition to determining the containment level These equivalences between test levels only consider the maximum transverse kinetic energy that the safety barrier is capable of absorbing during the impact.
Systems considered equivalent do not behave in exactly the same manner, and in each case, other factors such as working width, dynamic deformation, and severity level must be analyzed to select the most appropriate system according to the site conditions.
Severity Levels Under the EN 1317 standard, the tests of safety barriers with containment levels T3, N2, H1, H2, H3, H4a, and H4b, in addition to determining the maximum containment level of the system, must verify that the device is not so rigid a unit as to cause severe injuries to the occupants of a light vehicle.
To evaluate the level of deceleration of the vehicle during the impact, indicators obtained from the records of accelerometers installed inside the vehicle, near its center of gravity, are used. These indicators are:
. Theoretical Head Impact Velocity (THIV) . Post-Impact Head Deceleration (PHD) . Acceleration Severity Index (ASI) The severity indices must conform to the requirements specified in Table 6. Three levels of severity are established based on the THIV, ASI, and PHD indices.
Level A offers a higher degree of safety to vehicle occupants than Level B, and Level B offers a higher degree of safety than Level C. All else being equal, it is preferable to install a Level A system.
In turn, the NCHRP Report 350 standard establishes two parameters as criteria for evaluating the level of severity for vehicle occupants: the impact velocity of the occupants and the negative acceleration experienced during full-scale tests.
Table 7 shows the maximum allowable velocity values during full-scale tests, while Table 8 indicates the criteria corresponding to negative acceleration.
Furthermore, as an additional measure of the potential risk to vehicle occupants, additional measurements can be performed with an instrumented Dummy of the Hybrid III Dummy type (which is valid only for measuring frontal and head impacts, in which the movement is essentially parallel to the longitudinal axis of the vehicle). For more details, it is suggested to review NCHRP Report 350 and Chapter V of the Code of Federal Regulations of the United States.
NCHRP Report 350 does not require measuring or calculating the THIV, PHD, and ASI indicators. However, for the acceptance of vehicle restraint systems in Costa Rica, the SCV Manual recommends that these indicators must be reported in the test results and comply with the criteria of Table 6.
Deformation of the System According to its working width, the EN 1317 standard classifies the deformation of safety barriers according to the criteria shown in Table 9.
The EN 1317 standard requires that the tire track of the vehicle remain inside a zone called the "CEN Box" or, if the vehicle crosses this zone, it does so at a speed less than 10% of the nominal test speed.
According to the NCHRP Report 350 standard, the test report must include the dynamic deflection data (maximum lateral deformation the system undergoes during the impact) and the permanent deflection of the system (lateral deformation the system presents after the crash). However, this standard does not classify the system based on its deformation.
For the purpose of standardizing and classifying restraint systems based on their deformation, the SCV Manual has adopted the working width classification of the European standard EN 1317.
Redirection Capacity The EN 1317 standard evaluates the redirection capacity of a system by means of the CEN Box ("CEN Box"), which is represented in Figure 3.
If the vehicle's wheels after the impact cut a theoretical parallel segment located at a certain distance from the system, then the barrier is considered to lack redirection capacity and is not acceptable.
The criteria on the redirection capacity of the system in the NCHRP Report 350 standard correspond to evaluation factors K, M, and N in Table 10 (Vehicle trajectory).
Regarding this performance parameter, the SCV Manual has not adopted any of the criteria in particular, but requires that the restraint system comply with the criteria corresponding to the test standard used by the manufacturer to evaluate the restraint system.
6. The Procedure This section summarizes the procedure for analyzing and designing the margins of a road from the road safety perspective of the SCV Manual.
The general analysis and design procedure consists of three stages:
. Margin analysis.
. Margin improvement.
. Implementation of a vehicle restraint system.
6.1 Margin Analysis The procedure is oriented toward determining whether the margin of a road in a given section presents conditions such that the consequences of a run-off-road crash are not serious. For this condition to be met, it is necessary that the margin is free of potentially hazardous obstacles and that the terrain has a relatively flat cross-section.
The first stage consists of determining whether there is a clear zone (zona libre) free of potential hazards available on the roadside margin, for which one must:
. Perform an inventory of the elements located on the road margins, measure their dimensions and location relative to the highway travel lanes, and establish which ones could be potentially hazardous (for road users or third parties) due to their dimensions and characteristics.
. Measure widths and slopes of the terrain on the roadside margin.
. Establish the available clear zone (zona libre disponible, ZLD).
. Calculate the necessary clear zone (zona libre necesaria, ZLN) for each section of the road.
. If the available clear zone (ZLD) is greater than or equal to the necessary clear zone (ZLN), the roadside margin is considered safe, and it is not necessary to implement any measure.
The Clear Zone The clear zone (zona libre) is the space located on the roadside margin in which, after leaving the road, a driver could redirect or stop their vehicle safely, without overturning, colliding against any hazardous obstacle, or causing harm to a third party.
The necessary clear zone (zona libre necesaria, ZLN) is the distance measured from one of the edges of the road toward the corresponding margin, necessary so that, after leaving the road, a driver can redirect or stop their vehicle safely (without overturning or colliding against any hazardous obstacle).
The available clear zone (zona libre disponible, ZLD) is defined as the area between the edge of the road and the closest obstacle, steep slope (desnivel), or vulnerable object to it (See Figure 4).
The following sections present the technical criteria that allow establishing the necessary and available clear zones.
Slopes and the Clear Zone Fill slopes (taludes de relleno) parallel to the road are classified according to their gradient (pendiente) in accordance with the criteria of Table 11.
Flat terrains and those considered traversable and recoverable would allow a driver of a vehicle that leaves the road to travel safely and regain control of the vehicle, or come to a complete stop to then return to their travel lane on the highway.
If the terrain gradient is classified as acceptable—traversable but not recoverable—a vehicle that leaves the road will probably not overturn when traveling over the slope (talud), but depending on the speed at which it is traveling, it would not be possible for it to stop in that zone, and it will descend to the end of that incline.
On the other hand, if the slope gradient (pendiente del talud) is classified as critical—non-traversable—the vehicle runs the risk of overturning.
Preferable slope zones are considered safe and suitable for high-speed highways and high volumes of heavy vehicles.
Fill slopes (taludes de relleno) parallel to the road are classified according to their gradient in accordance with Table 12. In this case, two factors are considered: the gradient and the height of the slope (talud).
Calculation of the Necessary Clear Zone The minimum necessary clear zone (zona libre mínima necesaria, ZLMN) is the minimum width of the necessary clear zone, assuming the terrain on the roadside margin is flat. This theoretical reference parameter is obtained through the following equation:
ZLMN = ZLMN0 * FC (Equation 1) Where:
ZLMN0 = value of ZLMN for sections with straight horizontal alignment. This value is obtained from Table 13, depending on the design speed and the traffic volume (TPDA in both directions of travel) of the highway section.
FC = is the correction factor due to the road's curvature radius. It is obtained from Table 14.
The values specified in Table III-3 are applied as ZLN to straight road sections in which the roadside margin has slopes with a negative gradient equal to 1V:6H or flatter, or slopes with a positive gradient less than 1V:2H. In the case of curved sections, these values must be corrected by the factors FC from Table III-4 according to the curvature radius.
When the slopes (taludes) located within the ZLMN have gradients greater than 1V:6H, the ZLN is greater than the ZLMN because an additional area is required for a vehicle to maneuver and recover or to stop.
Figure 5 and Table 15 show the general criteria for determining the ZLN on fill slopes (taludes de relleno), based on the ZLMN and the topography of the slope (talud).
Ditches and Channels Ditches (cunetas) and channels must be designed to evacuate the design surface runoff and additional water in excessive rains, with minimal flooding or damage to the travel lanes. However, these channels must also be designed, constructed, and maintained considering their effect on road safety.
Figure 6 and Figure 7 show the preferable and acceptable gradient combinations for triangular and trapezoidal ditches and channels, respectively.
Provided that economic or space constraints do not prevent it, the inclination of the ditch walls must be equal to or less than 1V:6H, and their edges rounded with a minimum radius of 10 m. Ditches that meet these design criteria are called safety ditches (cunetas de seguridad). Figure 8 shows a safety ditch.
Reduced ditches, similar to the one shown in Figure 9, should only be used when the terrain is rugged and there are space limitations, and they must be covered with a grating or slotted lid, or be protected by a safety barrier.
6.2 Roadside Improvement If the roadside does not possess conditions to be considered safe (the clear zone (zona libre) is not sufficiently wide), whether due to the existence of obstacles, terrain slope, or other potential hazards, the roadside must first be improved by eliminating or modifying obstacles to reduce the risk they produce, and by attempting to widen the available clear zone (zona libre).
As a last resort, if it is not possible to modify the roadside, the possibility of installing a vehicle restraint system should be considered.
Identification and Treatment of Potential Hazards The risk associated with an element depends on the probability of it being impacted by a vehicle leaving the road and the severity of the collision (accident severity).
The general obstacle identification procedure is shown in Figure 10.
Elements classified as potentially hazardous must be treated to guarantee the safety of road users and other vulnerable third parties located on the roadsides.
7. Vehicle Restraint System Design Procedure It is important to be clear that the implementation of a vehicle restraint system (VRS) (sistema de contención vehicular, SCV) is not necessary in all cases, as it is adopted as a final solution when the safety problem cannot be resolved with any measure from the roadside improvement stage.
If it is necessary to install a vehicle restraint system (sistema de contención vehicular), the analysis and design procedure allows establishing the necessary containment level of the system, as well as designing the system, that is, establishing the system layout parameters (length, flare (esviaje), lateral offset, etc.).
7.1 Procedure Summary Figure 11 shows the general procedure for the design of a road safety barrier, with "design" understood as the methodology to determine the technical characteristics of the barrier —containment level, working width (W), maximum deflection (D), terminal type— and the value of the parameters for its layout —length, lateral and height location, flare (esviaje).
This procedure can be applied for the design of safety barriers that will be installed on both existing roads and planned roads.
7.2 Selection of the Containment Level Table 16 shows the accident severity classification according to the conditions of the potential hazard.
Once the severity level of a possible run-off-road accident is defined, and based on the speed of the road section (design speed or V85, as applicable to a planned or operational road), the average daily traffic (ADT) (tránsito promedio diario, TPD), and the average daily heavy vehicle traffic (tránsito promedio diario de vehículos pesados, TPDp), the barrier containment level is chosen according to the criteria in Table 17.
Once the severity level of a possible run-off-road accident is defined, and based on the speed of the road section (design speed or V85, as applicable to a planned or operational road), the average daily traffic (ADT) (TPD), and the average daily heavy vehicle traffic (TPDp), the barrier containment level is chosen according to the criteria in Table 17.
7.3 Lateral Location of the Barrier Distance to the Edge of the Pavement Safety barriers must be placed at a minimum separation from the edge of the traveled way of 0.50 m, and if possible, placed beyond the distance of concern (LS, see Table 18).
If the road has a shoulder, safety barriers will be placed outside of it. In any case, it is recommended to place safety barriers as far as possible from the edge of the road, but without exceeding the maximum distances indicated in Table 19).
Distance to Obstacles and Embankments The following criteria establish the minimum distances between a safety barrier and the potentially hazardous element (See Figure 12):
. The distance between a rigid object and the safety barrier must be greater than the working width (W) of the system to prevent vehicles from being snagged by the obstacle.
. The distance between the safety barrier and a critical slope, embankment, or body of water must be greater than or equal to the dynamic deflection (D) of the system, and never less than 0.5 m.
. If the restraint system (sistema de contención) is placed on a bridge deck, on a retaining wall, or at the edge of a cliff, the barrier must be rigid and no type of vehicle may pass over it or lean in such a way as to overturn and fall into the precipice. For this reason, safety barriers of the "bridge parapet" type must be used in these cases.
Height Layout The recommended height for each road restraint system, safety barrier, or bridge parapet is established by the manufacturer, in accordance with prototypes successfully full-scale crash tested under the EN 1317 standard or the NCHRP Report 350 standard, which are performed to approve and classify a vehicle restraint system (sistema de contención vehicular).
The reference point from which the barrier height must be measured is specified in Figure 13.
If the lateral distance between the outer limit of the lane and the vehicle restraint system (sistema de contención vehicular) is less than or equal to 2.0 m, the height is measured with respect to the outer edge of the lane.
If the distance is greater than 2.0 m, the height is measured with respect to the ground surface, at a distance of 0.5 m from the front face of the barrier.
Barrier Length The variables considered in the methodology for calculating the length of the barrier section before the obstacle are shown in the following Figure 14 (obstacle approach section).
LR = is the theoretical distance traveled by an out-of-control vehicle leaving the road before stopping. It is measured parallel to the road from the start point of the hazardous zone to the point where the vehicle is assumed to leave the road. This parameter is obtained from Table 20 based on the speed of the road section (design speed or V85) and its ADT (TPD).
ZLN = is the width of the necessary clear zone (zona libre necesaria).
LA = is the lateral distance from the edge of the travel lane to the farthest point of the obstacle or hazardous zone. If the hazardous zone extends beyond the limit of the necessary clear zone (ZLN), LA can be considered equal to the width of the necessary clear zone (LC) for calculating the length of the safety barrier.
LO = is the length of the obstacle measured parallel to the road.
L1 = is the length of the barrier section parallel to the road before the obstacle, and its value is determined as follows:
L1 = 0 if the obstacle does not protrude from the ground, for example: non-traversable slopes, bodies of water.
L1 = 8 m if the obstacle protrudes from the ground, for example: trees, posts, bridge piers, drainage system structures, and others.
L1 = 5 m minimum for bridge parapets.
L2 = is the lateral distance from the edge of the road to the section of the safety barrier parallel to the road.
L3 = is the lateral distance from the edge of the road to the obstacle or hazardous zone.
b:a = is the flare rate (razón de esviaje), which is determined based on the speed of the road section (design speed or V85), the type of system, and the location of the system with respect to the edge of the road. The type of system refers to its classification according to the rigidity level per Table 21. The flare rate (razón de esviaje), b:a, is established according to the criteria in Table 22.
X = is the length of the barrier section before the obstacle. If the barrier is placed parallel to the road along its entire length, X is calculated using Equation 2, and if the barrier is installed with a flare (esviaje), X is calculated using Equation 3.
Y = is the lateral distance from the edge of the road to the start or end of the barrier. Y is calculated using Equation 4.
The length of the barrier section after the obstacle is calculated following the same methodology proposed, but the design variables are measured with respect to the edge of the travel lane in the opposite direction (see Figure 15).
Barriers on Curved Road Sections The length of the barrier on a curved road section is calculated using a graphical methodology. It is assumed that the vehicle's departure path from the road is tangent to the curve. This will be the case if the clear zone (zona libre) available on the roadsides is flat and traversable (slopes equal to 1V:3H or flatter).
A line must be drawn from the outer edge of the obstacle or the limit of the clear zone (zona libre) to a point of tangency on the curve to determine the barrier length, as shown in Figure 16. Generally, it is not required to move the terminal away from the edge of the road (flaring effect (efecto de esviaje)).
Barriers in Medians A vehicle restraint system must be installed in the median of a road under the following conditions:
. A risk analysis or the current criteria (Figure 17) indicate that there is a high probability of vehicles crossing the median and suffering a head-on collision with other vehicles traveling in the opposite direction.
. Analysis of accident records shows it is a hazardous zone.
. Non-traversable slopes, in accordance with the criteria of Section 2 of this Chapter III.
. Potentially hazardous fixed objects, such as luminaires, bridge piers, culverts, are located within the necessary clear zone (ZLN) in the median (see criteria for defining the ZLN (clear zone necessary) in Section 2 of this Chapter III), and for some technical or economic reason, it is not possible to remove them, relocate them, or modify said objects to make them "traversable"—for example, by replacing post bases with collapsible systems.
The graph in Figure 17 is a guideline for determining whether there is a high risk of a vehicle crossing the median and suffering a head-on collision with another vehicle traveling in the opposite direction. Based on the ADT (TPD) (average daily traffic in both directions of travel) and the median width, the procedure that the professional responsible for the administration or design of a road must follow is established.
These criteria apply only in cases where the median has traversable terrain and there are no obstacles, such as fixed objects, in the median.
If the risk is indicated as minimal, the placement of the safety barrier is optional; however, the road design should facilitate the installation of the barrier in the future if the traffic volume increases significantly or a high accident rate occurs.
If an investigation is required, a benefit-cost analysis or a risk assessment must be performed, considering factors such as traffic volumes, vehicle fleet composition, accident history, median topography, and horizontal and vertical alignment.
If a vehicle restraint barrier (barrera de contención vehicular) must be placed, the containment level is selected according to the criteria in Table 23.
If the exposure level to traffic accidents is high, installing a very high containment barrier, type H4b, should be considered.
Three types of medians are established:
Type I: Medians with a swale-type cross-section.
Type II: Medians separating travel lanes at different elevations.
Type III: Elevated medians, where the terrain slopes form a cut section.
If the slopes forming the swale are equal to 1V:3H or steeper, a vehicle restraint barrier must be placed on both sides of the median, as shown in Figure 18, Illustration 1.
If one of the slopes forming the swale has a slope equal to 1V:3H or greater (steeper) and the slope of the other side is less than 1V:3H (flatter), a barrier must be placed to protect users who are at risk of overturning when descending the non-traversable slope (greater than or equal to 1V:3H), as seen in Figure 18, Illustration 2.
If the slopes forming the swale are less than 1V:10H (flatter) and the risk of a vehicle crossing the median and suffering a head-on collision is high (according to the graph in Figure 17), a barrier must be placed at the center of the median, unless other potentially hazardous fixed objects are located there. Figure 18, Illustration 3 shows this case.
If the slope forming the median is greater than 1V:10H (steeper) and the risk of a vehicle crossing the median and suffering a head-on collision is high (according to the graph in Figure 17), a barrier must be placed to protect users who are at risk of descending the slope and invading the travel lanes in the opposite direction, as shown in Figure 18, Illustration 4.
If the slope surface is rough, rocky, irregular, or unstable, a barrier must be placed on both sides of the median, as seen in Figure 18, Illustration 5.
If the slope is equal to 1V:10H or flatter and the risk of a vehicle crossing the median and suffering a head-on collision is high (according to the graph in Figure 17), the barrier must be placed in the center of the median, as seen in Figure 18, Illustration 6.
If the cut slopes of the median present a rough, rocky, irregular, or unstable surface, a barrier must be placed on both sides to protect road users. Otherwise, no safety barrier is required.
8. Crash Cushions (Atenuadores de Impacto) 8.1 Implementation Criteria The installation of a crash cushion is justified whenever the distance from a discontinuous rigid obstacle to the edge of the road or any other road reference point is less than that recommended on the roadside or median (according to the necessary clear zone (ZLN) criteria of Section 2 of this Chapter III) and it cannot be protected against a head-on impact by the implementation of safety barriers.
The installation of crash cushions is specifically justified in the following cases: "Gores" on exit ramps. When a hazardous zone associated with an exit divergence or fork does not have a flat area free of obstacles of at least 60 m from the opening point of the diverging lanes, a redirective crash cushion (atenuador redirectivo) must be provided (see Figure 19).
In the "gores" of an exit ramp, the use of safety barriers with curved beams or fences joining two barrier alignments, as well as frontal flares converging at a point, shall be avoided.
Median beginnings. When the start of the double safety barrier in the median is less than 40 m from the first obstacle located within it, a redirective crash cushion (atenuador de impacto redirectivo) must be provided (see Figure 20).
8.2 Selection of the Containment Class To determine the containment level of a crash cushion (atenuador de impactos), the design speed or V85 of the road section where it will be installed must be considered, as the class or containment level of these systems is specified in terms of operating speed, and four classes exist: 110 km/hr, 100 km/hr, 80 km/hr, and 50 km/hr.
Table 24 shows the criteria for choosing the containment class of an Impact Cushion.
9. Terminals 9.1 Selection of Terminal Type The ends of a safety barrier cannot, in themselves, constitute a potential hazard to road users.
The most recommendable and natural type of terminal for a safety barrier is its embedment in a slope. Whenever site conditions allow it, this type of terminal must be used for the ends of safety barriers (see Figure 21).
The embedment of the barrier ends must guarantee the anchorage of the barrier, the stretch from the barrier to the embedment must maintain the appropriate height, and the flare angle (ángulo de esviaje) must comply with the criteria in Table 22.
When it is not possible to anchor the ends of the barrier, either due to the lack of a slope for this purpose, lack of space, or the existence of other interposed elements, then it will be necessary to resort to another type of barrier terminal.
Abrupt (unprotected) terminals must be definitively excluded due to their clearly negative behavior at any speed.
From the point of view of their behavior upon vehicle impact, energy-absorbing terminals (TAEs) are always of superior performance and, therefore, are preferable to flared (ground-anchored) terminals (terminales en abatimiento) in any application.
When it is not possible to anchor the ends of the barrier, either due to the lack of a slope for this purpose, lack of space, or the existence of other interposed elements, then it will be necessary to resort to another type of barrier terminal.
Abrupt (unprotected) terminals must be definitively excluded due to their clearly negative behavior at any speed.
From the point of view of their behavior upon vehicle impact, energy-absorbing terminals (TAEs) are always of superior performance and, therefore, are preferable to flared (ground-anchored) terminals (terminales en abatimiento) in any application.
Given that the widespread installation of TAEs is still not very feasible today for economic reasons, it is convenient to determine in which situations a TAE guarantees a higher benefit/cost ratio. Replacing a flared terminal with a TAE is more beneficial the greater the risk of vaulting and rollover.
Two particular situations are indicated below where the use of flared barrier terminals (terminales de barrera en abatimiento) can represent a hazard:
Flared terminal close to the road and high risk: a flared barrier terminal (terminal de barrera en abatimiento) parallel and very close to the road edge (see Figure 22) generates the risk that, when impacted frontally or laterally, it causes the vehicle to vault and rollover. The consequences of this type of accident can be severe. This risk increases with speed, proximity to the edge, and curved alignment.
In the case of a flared terminal with high risk due to proximity, it is convenient to install the barrier in such a way that, in plan view, it presents an angled or flared (esviaje) section, so that the buried end of the flared terminal moves away from the road edge (see Figure 23). Table 22 contains the recommended flare rates (razones de esviaje) (b:a).
In the case of a flared terminal with high risk due to proximity, it is convenient to install the barrier in such a way that, in plan view, it presents an angled or flared (esviaje) section, so that the buried end of the flared terminal moves away from the road edge (see Figure 23). Table 22 contains the recommended flare rates (razones de esviaje) (b:a).
Terminals at bifurcations: a zone of special interest for TAE implementation is exit branches, bifurcations, or divergences when they present either a single barrier alignment parallel to one of the separating roads (Figure 24) or two converging safety barrier alignments, one parallel to each road (Figure 25).
The case of a single barrier alignment (Figure 24) parallel to one of the separating roads will occur when the hazardous zone only affects one of the roads (generally, the main road). In this case, the implementation of an energy-absorbing terminal (TAE) is recommended.
The case of two barrier alignments (Figure 25) parallel to each road respectively and converging towards a point will occur when the hazardous zone justifying the barrier implementation affects both separating roads.
When the slope between both platforms is less than 2:1 (flatter), it is recommended that the barrier for the secondary road starts from the section where the edges of said platforms are at a minimum distance of 3 m.
In the case of two barrier alignments, respectively parallel to each road and converging at a point (when the distance between the ends is less than 3 m), it is necessary to resort to the implementation of a redirective crash cushion.
The use of a TAE in bifurcations where barriers exist close to the divergence point is recommended both in the case of a single barrier alignment and in the case of two converging alignments with ends sufficiently separated.
9.3 Selection of the Containment Level Regarding the selection of the containment level for energy-absorbing barrier terminals (TAEs), the same criteria established for crash cushions can be applied, excluding the 50 km/hr class, which is not defined for terminals.
Table 25 shows the criteria for choosing the containment class for a TAE.
10. Transitions When two barrier sections with different behavior (containment level or deformation class) are connected longitudinally, an intermediate section or transition must be provided, which is considered a safety barrier with some particularities or reservations regarding the critical point and direction of impact.
Table 26 establishes the criteria for selecting the containment level for the transition between two safety barriers.
In transition sections, whether between barriers of the same or different containment levels, not only the containment level must be considered, but also the difference in deformability between the barriers connected longitudinally.
The transition from a more deformable barrier to a more rigid one according to the direction of impact—which is the problematic case—can cause a light vehicle to be snagged at the transition point. Snagging is an accident with serious consequences. To verify that this difference in deformabilities is not hazardous, it is necessary to compare the dynamic deflection (D) of both barriers corresponding to the TB11 test—except in the case where both barriers have an N2 containment level, in which case their dynamic deflections from the TB32 tests would be compared.
11. Conclusions . There is an indisputable need for a technical guide that defines uniform, valid, and official criteria for the analysis of road safety on the roadsides of national highways.
. The only technical guide existing in the country related to roadside safety is that contained in provision MN-06-2006 "Flex Beam Type Steel Barrier (Barrera de Acero Tipo Viga Flexible)", which regulates the selection of materials and installation of guardrails (guardavías) in our country. It focuses on standardizing the technical aspects for the supply and installation of flex beam type steel barriers, leaving out other types of vehicle restraint systems available on the market that would meet the needs for roadside improvement and treatment.
. Provision MN-06-2006 includes some technical criteria set out in other manuals, but does not refer to the evaluation and treatment of obstacles on roadsides, the sizing of the clear zone (zona libre), and the selection criteria for the type of system, such as containment level and barrier deflection.
. The preliminary version of the "Manual for the analysis of road safety on roadsides and the layout of vehicle restraint systems of Costa Rica" prepared by the technical team responsible for this study is a good guide that includes the necessary technical aspects to perform a good analysis of road safety conditions on roadsides, and the design of solutions (including roadside improvement and the selection and layout design of vehicle restraint systems).
. Regarding the technical aspects of the guide, it is concluded that the criteria and procedures of the SCV Manual are generally appropriate and applicable to the conditions of the national reality. However, it was determined that the AASHTO (2006) criteria—which were included in the preliminary version of the SCV Manual for use in Costa Rica—for determining whether the cross-section of a ditch (cuneta) or swale is traversable are not consistent with the criteria applied in the case of ascending slopes, and on the other hand, these AASHTO (2006) criteria tend to classify as non-traversable some existing ditch (cuneta) sections on our roads, whose design is not potentially hazardous according to the professional judgment of the professional team that participated in the execution of this technical study.
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