The aim of the study was to compare the safety effects of studded and unstudded winter tires based on fatal road accident data collected by road accident investigation teams. The data included 958 road accidents involving a passenger car or van that occurred in Finland from November to March between 1997 and 2012. However, cases categorized as suicides or medical events and those in which the driver had fallen asleep were not included. The study was limited to drivers who had the most decisive effect on accident occurrence (A-party).
The main results show that the more slippery the road, the more likely unstudded tires were to be used (figure 1). The state of repair was less pertinent for unstudded than for studded tires (figure 2).
In addition, drivers using unstudded tires were more frequently intoxicated or exceeded the posted speed limit more often by more than 20 km/h. On the other hand, drivers using unstudded tires were more experienced on average. When evaluating the wholeness of the results, it is clear, that the differences in driver behaviour do not explain, why unstudded tyres are more often in A-party cars in slippery conditions. Therefore, it can be assessed, that fatal accident risk in icy conditions is higher with unstudded winter tyres.
Comparison of accident frequency and proportion of tire types in traffic showed that the use of unstudded tyres alone would have led to an annual increase of nine fatal road accidents in Finland. However, the effect was not statistically significant. Overall, it was assessed that the risk of fatal road accidents on icy roads is higher for unstudded than for studded tires. However, the magnitude of this risk difference is difficult to determine without specific exposure information.