The aim of this study was to find an adequate method of estimating the road safety effect of the use of different types of tyres. One problem when studying the road safety effects of different tyres is that the type of tyre used is not independent of other variables influencing road safety. One such example is young drivers, who are more accident-prone and who do not use studded tyres to the same extent as older and more experienced drivers. If these interface variables are to be considered, a considerable amount of data is required. On bare ground there should not be any effect of studded tyres worth mentioning. If the measured relative risk of studded tyres/no studded tyres on bare ground deviates from 1, the deviation originates from the influence of other variables. By standardising the measured relative risk in slippery conditions with the relative risk on bare ground, a measure of the relative risk in slippery conditions will be obtained where all interface variables have been considered. The same number of vehicles is still needed but the amount of collected data can be reduced considerably. Information about cars in accidents including the type of tyre was given by the police. Information on control vehicles was covered via questionnaires to find, for example, the vehicle mileage of passenger cars with different tyres in different road conditions in the different Swedish counties. Different ways to calculate the traffic safety effect are described. (A)
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