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The UK’s roads are amongst the safest in Europe, nonetheless the number of accidents involving vehicles leaving the carriageway remains high when considered as a proportion of all accidents. Indeed in 2009 fatalities as a result of a single vehicle run-off represented nearly half of all UK road fatalities.
Despite the large numbers of accidents nationally the number of incidents of a vehicle leaving the carriageway at one particular site is likely to be low. Justifying the introduction of expensive Road Restraint Systems (RRSs) to reduce the risk is a challenge for local highway authorities, especially at a time when funding for maintenance and improvements scheme is already limited. Authorities must be confident that any measures taken represent good value for money.
The Design Manual for Roads and Bridges standard TD 19 – Requirements for Road Restraint Systems has been developed using accident data for routes with over 5000 AADT and a speed limit of 50mph or greater. Application of the risk based approach in that standard to low speed and low flow roads is likely to result in over use of RRSs and not represent best use of limited resources. TD 19 is therefore not suitable for use on the majority of the nation’s local road network.
It is not possible to produce a prescriptive set of standards to govern the use of RRSs on local authority roads. This Guidance Document provides the outline of an appraisal process to help authorities decide when a RRS is justified. This appraisal takes account of the many diverse influencing factors including risk assessment, alternative solutions, system feasibility, cost benefit analysis and the availability of funding.
The Guidance contained here can be adapted by local highway authorities to create a pragmatic system for decision making to help them make best use of the finite resources available to them.