The skidding resistance standard for trunk roads in England is contained in HD28 in the Design Manual for Roads and Bridges. It requires an Investigatory Level of skidding resistance to be assigned for each part of the network, based on the site category (carriageway type and geometry etc.) plus other local factors. If the measured skidding resistance falls to, or below, this level a site investigation is triggered to determine if resurfacing the road would reduce accident risk. This report describes the analysis of accident risks that was undertaken as part of an update to the standard made in August 2004. The accident analysis was made in three parts. Firstly, accident risk was compared to skidding resistance for the site categories in the previous version of the standard. Secondly, generalised linear modelling was used to test the significance of skidding resistance and other factors, such as texture depth and road geometry, on accident rates. Finally, new site categories and Investigatory Levels were recommended. The range of accident risk at sites within the same category is large and as a result a range of possible Investigatory Levels has been recommended. It was also recommended that the advice in the standard on setting Investigatory Levels and subsequent site investigation is strengthened. A simple estimate of the financial costs and benefits of applying the recommendations was also made. It appears that in adopting the recommendations of this report, Highways Agency will both reduce accident risk on trunk roads in England and that the costs of resurfacing that result will be recovered in the financial value of accidents saved. (Author/publisher)
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