During periods of frost and snow, the formation of a layer of ice on a road surface will lead to a large reduction in friction, and increase significantly the risk of skidding accidents. For this reason, highway authorities have a duty to keep highways free of ice and snow and endeavour to meet this duty through winter service operations. However, concerns have been expressed by some road users that de-icing operations, particularly repeated operations when de-icers can build up on the road surface, can adversely affect skid resistance.
The Highways Agency commissioned TRL to investigate the effect of de-icing operations on skidding accidents and skid resistance. This report describes a desktop study to address the first objective of the research project: to identify where and when a reduction in skid resistance due to repeated de-icing operations has been experienced on the Agency’s network and any common factors.
The data were analysed in three stages:
1. an overview of winter accidents in comparison with other times of year using the national accident data;
2. comparison of the accident densities and characteristics of winter accidents on an Area basis with and without the use of de-icers using the Highways Agency’s database that reports winter service operations in combination with the accident data;
3. comparison of the accident rate on routes on days with and without the use of de-icers. This analysis used traffic data in addition to the data used in the second stage.

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