Crash Barrier

Thu, 2010-09-02 20:29

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Standard guardrail (A-profile)

A crash barrier or guardrail is a barrier on a road designed to prevent vehicles from leaving the roadway to improve road safety. Common sites for crash barriers are:

median separators on multi-lane highways

bridge supports

mountain roads

The design of the road barrier is generally such that a vehicle hitting the barrier is steered back onto the road. This may be achieved by designing the supports so that they break off on impact, allowing the barrier to deform and push the vehicle back on track. In some cases cost cutting has led to a failure of this mechanism, with so-called "duck-nesting" (after the shallow nature of a duck nest) of barrier support bases. When this happens the supports tilt over at the base instead of breaking off, allowing the barrier to collapse and the vehicle to go over the barrier. Motorcycles are very vulnerable to crash barriers. Large vehicles with a high centre of gravity, such as sport utility vehicles, are also vulnerable to going over barriers on impact.

To prevent heavy vehicles going through or over the barrier and still maintain a low impact severity level, more stable systems like the German Super-Rail have been developed since the 1990s. It is tested to hold up and lead back trucks up to 40 tons while causing as low damage to smaller vehicles as the standard system of the 1930s.

Super-Rail Crash Barrier left; on the right: standard spaced guardrail

With effect from January 2005 and based primarily on safety grounds, the UK Highways Agency's policy is that all new motorway schemes are to use high containment concrete step barriers in the central reserve. All existing motorways will introduce concrete barriers into the central reserve as part of ongoing upgrades and through replacement as and when these systems have reached the end of their useful life. This change of policy applies only to barriers in the central reserve of high speed roads and not to verge side barriers. Other routes are said to continue to use steel barriers, however there have been concrete barriers installed on major A-roads as well.