Vehicle restraint systems
Types of vehicle restraint systems, testing, installation, maintenance and considerations
The main aim of vehicle restraint systems (VRS) is to absorb and decrease the force of a crashing vehicle. It does this through deformation, or displacement, reducing the severity of the accident by effectively limiting the damage to the occupant(s) of the vehicle(s) involved. It also protects the surrounding environment which includes buildings, road furniture, trees and embankments from erratic vehicle movement.
In UK and Europe, vehicle restraint systems must be tested and certified to the EN 1317 standard. This is verified by external accredited bodies and all suppliers of vehicle restraint systems are audited on an annual basis.
A qualified civil engineer oversees the design of vehicle restraint systems. As part of this design the engineer considers the road infrastructure management, with the aim of preventing or minimising crashes due to vehicles leaving the carriageway.
The design is based on the main principle of bringing an errant vehicle back into the carriageway. This reduces injury or fatal crashes. The second priority is to reduce the severity of the crash. The resulting design embraces various vehicle restraint systems. These include safety barriers, wire rope systems, crash cushions, parapets, terminals and transitions. In the design, maintenance and repair costs must be specified and factored into the project.
What do vehicle restraint systems do?
A vehicle restraint system works on several levels:
- Initially it provides a visual indicator that overshooting the road is a risk.
- Next it deflects a colliding vehicle, returning it to a safe path.
- And finally, it sacrificially absorbs the impact momentum of the colliding vehicle.
The sacrificial elements require to absorb impact in a staged manner so as to restrain either a small car or a fully laden truck.
What factors to consider when implementing vehicle restraint systems?
When implementing a vehicle restraint system, consider:
- The layout of the road. e.g. single, dual carriageway, or motorway.
- The area to be protected is viewed as having different sections. e.g. curved, then straight, then narrowed. Each of these have different treatments.
- The composition and rate of flow of the traffic. e.g. vehicle types, quantity of type per day. With the peaks of both speed and volume, also taking into account the impact loads.
- The peripheral road risks factors. e.g. trees, road furniture, embankments, buildings and pedestrians. Including the distance from the traffic to the risk.
- Alternative risk treatment. e.g. removing, smoothing, or diverting an element from direct impact from the road.
What types of vehicle restraint systems (VRS) are available?
Safety barriers (Highway, Industrial and Off Highway)
Safety barriers are located at the edge of a road or in the central reservation. They prevent vehicles from leaving the carriageway and colliding with an oncoming vehicle or the hard surrounding infrastructure such as a concrete barrier. An example of safety barriers used by several UK suppliers is Flexbeam.
Wire rope systems
Wire rope systems are interleaved ropes which are located at the edge of a road or in the central reservation. They absorb the force of a vehicle with the aim of capturing, containing and redirecting the impacting vehicle. An example of wire rope systems used by a number of UK suppliers is Brifen.
Parapets
Parapets are restraint walls which acts as a barrier between vehicles (or people) and a sudden drop, such as bridge, embankment or walkway. Road works or on supporting walls can also use them. Like other barriers, they provide security against accidental collision. Also, in the case of a bridge parapet, they protect against a vehicle running off the road.
Terminals
Terminals are the end points of a safety barrier. Traditionally terminals were handled by having inclined starts, or curved sections. The best method of treating these, to avoid making a frontal collision worse, is to use crash cushions (or impact attenuators) instead.
Crash cushions or impact attenuators
Crash cushions, also known as impact attenuators, are collapsible devices. Their aim is to provide a way of absorbing frontal shock where there is a high risk of a collision occurring at a specific spot. Examples of this can be at the terminal ends of a section, such as the start of a central reservation or a bridge pillar surround.
Transitions
Transition protection ensures that the integrity of the crash barrier is not compromised at a connection between two barriers. This can be between two barriers of the same type, or of different systems.
When applying a transition it is an important engineering concern that the zone of transition absorbs the same level of impact as the two adjacent sections, so that a fail point is avoided.
Motorcycle Protection Systems
Motorcycle Protection Systems (MPS) are advancements of standard safety barrier design that reduce the risk to a two wheeled road user. So the main element is to avoid a fallen and sliding motorcyclist undershooting the barrier.
MPS are typically deployed to a parapet and a road edge barrier.
What standard of installation is required?
Both the product quality and that of its correct installation are vital to guarantee that a road restraint system is deployed to meet the standards for which it is designed for. That is, they are capable of reducing the damage as a result of an accident where a vehicle leaves the carriageway. In other words the installation must be efficient in reducing the potential deaths and injuries on the road. This requires a formal risk assessment to be conducted.
The installation of parapets systems can only be carried out by companies with NHSS10B accreditation and NVQ trained installer(s).
How are vehicle restraint systems tested?
Each component of a vehicle restraint system has to be produced by a supplier to a national or international standard. They must then be tested when deployed using a recognised test method.
The level of this deployment testing varies depending on the location used. For light traffic or pedestrian use, simple inspection by a suitably qualified inspector may suffice. However, in higher risk deployments this involves design simulation testing and physical stress testing. The posts foundations can also be tested on site by a push/pull test.
What level of maintenance is required?
Each deployment of a vehicle restraint system requires to have a formal inspection and test schedule defined. This may be on many levels, each assessed at different intervals. Visual inspections may be more frequent than structural testing for example.
A key aim of any inspection program is to assess that unreported damage to a system component is replaced as quickly as possible so that the integrity of the system is maintained.