This paper describes research at trrl to find the association of the type of weather with traffic flow and accidents, and to derive numerical factors which could be used to account for changes in weather. Economic consequences are assessed to justify the cost of counter-measures. The different road-weather conditions - rain, snow and ice, and fog are considered separately. It is not easy to compare results from a number of studies described because of differences in measurement frequencies and methods of measuring the weather, and differences in geographical locations. Bad weather reduces traffic flow and increases accident rates, but the magnitudes vary considerably according to the basic data used. The greatest weather problems are associated with wet roads. It is estimated that the increase in accidents in rain averages 52 per cent over the whole year, and on wet roads when it is not actually raining 50 per cent. Although fog, snow and ice are disruptive, the number of days affected each year are relatively few, and the proportion of accidents occurring in these conditions is small. The estimated average annual increase in costs of accidents due to bad weather is £46 million. This chapter was also published as trrl technical note tn776.
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