Inappropriate speed reduces the time available to react, increases the distance required to stop a vehicle and worsens the consequences of a run-off-road collision. When a driver loses control on a bridge, viaduct or road alongside a significant drop, the opportunity to correct the vehicle’s path can disappear within seconds.

At these critical locations, vehicle restraint systems provide the final physical line of protection between the carriageway and the hazard beyond its edge. Their purpose is not to prevent the collision from occurring, but to contain and redirect the vehicle so that it does not reach a drop, a road or railway below, a structure or another feature that could increase the severity of the incident.

During the August holiday period, long-distance journeys increase and factors such as fatigue, distraction, greater vehicle occupancy and inappropriate speed can occur together. Prevention must always begin with responsible driving, but the infrastructure must also be prepared to respond when human judgement fails.

Speed increases the energy that the system must manage

The potential severity of an impact does not depend on speed alone. The mass of the vehicle, the angle at which it strikes the barrier and the characteristics of the contact point are also involved.

Kinetic energy increases with the square of speed. This means that an increase in speed generates a proportionally greater increase in the energy that must be managed by the vehicle, the restraint system and its anchors.

A passenger car and a heavy vehicle do not present the same impact scenario. Although a lorry or coach may be travelling at a lower speed, its mass means that the energy and forces involved in the impact are considerably greater.

For this reason, the selection of a bridge parapet should not be based solely on the speed limit of the road. The volume of heavy traffic, the type of collision that could occur, the road geometry and the consequences of a vehicle passing through the system should also be assessed.

What is a high-containment vehicle restraint system?

Vehicle restraint systems are passive safety devices designed to intercept an out-of-control vehicle, absorb part of its impact energy and redirect its path towards the carriageway in a controlled manner.

UNE-EN 1317 establishes a common method for evaluating these systems through full-scale crash tests. The results determine parameters including containment level, impact severity for the occupants, working width, dynamic deflection and vehicle intrusion.

Within this classification, H2 and H3 are considered high-containment levels. They are intended for scenarios in which a vehicle leaving the road could have serious consequences and where the system must be capable of containing vehicles with a greater mass.

A system being classified as H2 or H3 does not mean that it is intended exclusively for roads with a particular speed limit. The level expresses the capacity demonstrated through a standardised combination of tests. The final selection should reflect the project’s risk assessment and the application criteria established for the infrastructure.

How is the H2 containment level demonstrated?

To achieve the H2 level, the system must pass two complementary tests. The first verifies its ability to contain a heavy vehicle, while the second assesses the safety of occupants in a light vehicle.

TB51 test with a coach

The principal H2 test is known as TB51 and is carried out under the following conditions:

  • Vehicle: coach.
  • Mass: 13,000 kilograms.
  • Impact speed: 70 kilometres per hour.
  • Impact angle: 20 degrees.

The test verifies whether the system contains the coach, prevents it from passing through or overturning over the barrier and redirects it without creating a dangerous path for other road users.

TB11 test with a light vehicle

The same system must also pass the TB11 test:

  • Vehicle: light passenger car.
  • Mass: 900 kilograms.
  • Impact speed: 100 kilometres per hour.
  • Impact angle: 20 degrees.

This second test verifies that a barrier capable of containing a heavy vehicle is not excessively rigid for a passenger car. It also allows the accelerations experienced by the occupants to be calculated and the impact severity level to be determined.

The META13® bridge parapet has undergone full-scale testing in accordance with UNE-EN 1317-2 and is classified at containment level H2. The test was carried out on a cantilevered slab to reproduce the conditions of a bridge deck.

What does the H3 containment level require?

The H3 level increases the containment capacity through a test involving a heavier goods vehicle and a higher impact speed.

TB61 test with a heavy goods vehicle

The characteristic H3 test is TB61:

  • Vehicle: non-articulated heavy goods vehicle.
  • Mass: 16,000 kilograms.
  • Impact speed: 80 kilometres per hour.
  • Impact angle: 20 degrees.

As with the H2 level, the system must also pass the TB11 test with a 900-kilogram passenger car travelling at 100 kilometres per hour. This verifies both its ability to contain the heavy vehicle and its compatibility with the safety of light-vehicle occupants.

The META16® bridge parapet has an H3 containment level and has undergone full-scale impact tests on a cantilevered slab. Its behaviour is defined through parameters such as working width, dynamic deflection, vehicle intrusion and impact severity.

What is assessed during a crash test?

A crash test does not simply verify whether the barrier remains standing after the collision. Accredited laboratories use high-speed cameras, sensors and measuring systems to study the complete behaviour of the vehicle, the restraint system and its components.

The criteria assessed include:

  • Whether the vehicle is prevented from passing through the restraint system.
  • Whether the vehicle is prevented from overturning as it passes over or climbs the barrier.
  • Whether the vehicle follows a controlled path after the impact.
  • Whether the vehicle is prevented from entering other traffic lanes dangerously.
  • Whether any large components become detached and create an additional hazard.
  • Whether the system’s deformation remains within the measured values.
  • Whether the accelerations transmitted to the occupants meet the acceptance criteria.
  • Whether the anchors and supporting structure perform as intended.

Passing the test allows the system’s performance to be characterised under repeatable conditions. It does not mean that every real-world collision will involve exactly the same mass, speed or angle, but it enables solutions to be compared using a common technical method.

Containing and redirecting: two inseparable functions

A high-containment system should not simply stop the vehicle abruptly. An extremely rigid barrier might prevent a vehicle from leaving the road but could transmit excessive deceleration to its occupants.

The system should deform in a controlled manner to absorb part of the energy and redirect the vehicle towards the carriageway. Achieving this balance between strength and deformation is one of the main challenges in bridge parapet engineering.

The vehicle’s subsequent path is also critical. It should not rebound uncontrollably or return to the carriageway at a perpendicular angle. Appropriate redirection reduces the possibility of the vehicle entering other lanes and causing a secondary collision.

Working width, deflection and vehicle intrusion

In addition to selecting H2 or H3, the designer must assess the space required for the system to deform and contain the vehicle.

Dynamic deflection

Dynamic deflection is the maximum lateral displacement of the face of the system closest to the traffic during the impact. It indicates how far the barrier moves from its original position.

Working width

Working width considers the maximum distance reached by any essential part of the system during the collision. This parameter is particularly important when noise barriers, sign supports, structural elements or pedestrian routes are located behind the parapet.

Vehicle intrusion

Vehicle intrusion measures how far the upper section of a heavy vehicle can extend during the impact. On a bridge, this can be critical when elements behind the parapet could come into contact with the coach or lorry.

A high containment level does not remove the need to provide these spaces. Placing an obstacle within the working width or vehicle intrusion zone can alter the performance demonstrated during the test.

The parapet and bridge deck operate as a complete system

During an impact, the forces generated by the vehicle are transferred from the rails and posts to the anchors and concrete structure. The parapet therefore cannot be assessed separately from the bridge deck.

The project must verify the geometry and strength of the slab, the reinforcement at the edge, the anchor type, the distance from the ends and the loads that may be transferred to the structure.

Installation conditions should reproduce the characteristics under which the system achieved its classification. Any significant change to the components, spacing, anchors or slab could affect its performance.

On existing structures, where it is not always possible to incorporate an embedded anchor plate during construction, alternative anchoring systems may need to be assessed and their equivalence under impact forces demonstrated.

When might an H2 or H3 level be required?

The selection should be based on a specific risk assessment and the criteria applicable to the road. Factors that may justify a high-containment system include:

  • The presence of bridges, viaducts or elevated crossings.
  • Significant drops alongside the carriageway.
  • A substantial volume of coaches or heavy goods vehicles.
  • Traffic passing over other roads, railway lines or populated areas.
  • The presence of structural components or critical installations.
  • The potential severity of a vehicle passing beyond the road edge.
  • The design speed and geometric characteristics of the route.

H3 should not automatically be considered a better option than H2 for every location. A higher-containment system may present different deformation, intrusion or transmitted loads. The appropriate solution is the one that addresses the identified risk and is compatible with the available structure.

The final line of defence when driving fails

Observing speed limits, resting properly, remaining attentive and adapting speed to road conditions are the first measures for preventing a run-off-road collision. No passive safety device can fully compensate for high-risk driving.

However, once control of the vehicle has been lost, the infrastructure has only a few moments in which to limit the consequences. On bridges, viaducts and roads alongside significant drops, a correctly selected and installed parapet can make the difference between a vehicle leaving the structure and being redirected in a controlled manner.

The TB51, TB61 and TB11 tests verify at full scale that H2 and H3 systems can manage impacts involving vehicles with very different masses and behaviours. This capacity must be accompanied by correct containment-level selection, assessment of working width and intrusion, and appropriate integration with the bridge deck and anchoring system.

Road safety does not depend solely on driver behaviour or exclusively on infrastructure. It is achieved through the combination of prevention, technical design, tested systems and installation that respects the conditions under which their performance was demonstrated.