Frequently Asked Questions (FAQ) on Vehicle Restraint Systems and Length of Need?

What is a Vehicle Restraint System (VRS)?

A Vehicle Restraint System (VRS) is a safety barrier designed to prevent vehicles from leaving the carriageway and colliding with roadside hazards. These systems include guardrails, barriers, and crash cushions, all engineered to contain and redirect vehicles upon impact.

What does “Length of Need” (LoN) refer to in VRS?

Length of Need (LoN) is the total minimum length of a VRS required to effectively contain and redirect vehicles. It ensures that the VRS provides adequate protection in advance of, alongside, and after a hazard to minimize the risk of vehicles breaching the barrier.

How is the Length of Need determined?

The Length of Need is determined by assessing the hazards that need protection, considering factors such as traffic speed, vehicle types, and road conditions. It involves calculating the required barrier length to ensure full containment and effective redirection of vehicles.

Why is the Length of Need important?

The Length of Need is crucial for ensuring that a VRS can fully contain and redirect vehicles, preventing them from colliding with roadside hazards. Proper determination of LoN is essential for maximizing road safety and minimizing the risk of injuries and fatalities.

What considerations are there for contraflow and dual carriageways?

In situations like contraflow and dual carriageways, where traffic can travel in both directions on the same carriageway, it’s important to assess whether the minimum Length of Need is sufficient for both normal and temporary conditions. The bidirectional traffic can alter the risk profile, requiring adjustments to the LoN.

What is the total length of a safety barrier?

The total length of a safety barrier includes the Length of Need plus additional lengths specified by the manufacturer. These additional lengths, positioned before and after the LoN, ensure that the safety barrier provides full containment and meets performance standards.

What is the significance of full containment?

Full containment ensures that vehicles are safely redirected without breaching the barrier, maintaining the integrity of the VRS during impact. Achieving full containment requires careful planning and consideration of site-specific conditions and potential hazards.

Where can more detailed guidelines on VRS and Length of Need be found?

For more comprehensive information on VRS and Length of Need, refer to the following resources:

• Design Manual for Roads and Bridges (DMRB) CD 377
• European Standard EN 1317
• Highways England Road Safety Measures

These documents provide in-depth guidelines and standards for the design, implementation, and maintenance of VRS, helping to ensure road safety.