Significant risk increase with winter tyres in summer

A new study from VTI, Folksam and the Tire Industry Information Council shows a sharply increased risk of fatal accidents when using winter tires during the summer.

It is well known that winter tyres are a safe choice on snow and ice. But the same properties that provide good grip in winter road conditions can become a disadvantage when the temperature rises and the roads are dry or wet. Researchers from VTI, Folksam and the Swedish Tyre Industry, in collaboration with the Swedish Transport Administration, have now investigated how the use of winter tyres during the summer affects the risk of fatal accidents in passenger car traffic.

The study is based on in-depth studies of all fatal accidents involving passenger cars in Sweden during the summer months of May to September between 2012 and 2021. The researchers analysed 725 fatal accidents and examined, among other things, tyre type, accident sequence and whether the road grip had likely affected the outcome of the accident.

The results show that passenger cars with at least one winter tyre were significantly overrepresented in the fatal accidents. While the proportion of winter tyres on Swedish passenger cars during the summer is estimated to be around seven per cent, 22 per cent of the cars in the studied accident material had at least one winter tyre.

The researchers believe that the overrepresentation can largely be explained by the winter tyres' poorer grip in summer conditions. The risk analysis shows that the use of at least one winter tyre resulted in a significant increase in the risk of a fatal accident where the grip had an impact on the course of the accident.

One explanation is that winter tyres provide longer braking distances on dry and wet asphalt during the summer. Previous tests have shown that the braking distance can be 20 to 40 per cent longer compared to summer tyres.

"Our results indicate that the large overrepresentation of winter tyres in fatal accidents during the summer is mainly related to the tyres' poorer grip on summer road conditions," says Mattias Hjort, researcher at VTI and lead author of the study.

The researchers also investigated several possible alternative explanations. Cars with winter tires were on average older and less likely to have anti-skid systems (ESC) than cars with summer tires. But even when the analyses were limited to newer cars, cars with ESCs or drivers without alcohol or drug intoxication, the increased risk remained.

Even in accidents where the car was driven within the current speed limit, a clear risk increase of at least 40 percent was found for cars with winter tires compared to cars with summer tires.

The researchers also compared the importance of tire choice with the effect of anti-skid systems. The increased risk associated with winter tires during the summer turned out to be of the same magnitude as the increase in risk that results from driving a car without an ESC.

The study also shows that road grip is important in a large proportion of fatal accidents. About a quarter of the fatal accidents involving passenger cars during the summer period were considered to be those where better road grip could probably have affected the outcome. The researchers therefore believe that the characteristics of the tyres should receive greater attention in future accident investigations.

"The importance of tyres has largely been overlooked in analyses of fatal accidents during the summer. Our results show that the choice of tyres can play an important role in road safety," says Mattias Hjort.

The study is also considered to be relevant outside Sweden. Winter tyres are used in many countries, and if their use during the summer is as common as in Sweden, there may be a road safety potential in more motorists switching to summer tyres when the winter road conditions are over.

Summer use of winter tyres and the effect on fatal passenger car accidents