The risk of road accidents increases as more and more roads become polished

In recent years, there have been a number of serious road accidents due to motorists skidding off the road. The reason is believed to be that the country’s roads are becoming increasingly smooth. VTI has investigated the cause of the problem in a pilot study. New designs of studded tyres, a rise in the use of friction tyres (winter tyres without studs), and increased traffic are just a few interconnected factors that may have an impact.

In 2018, skidding accidents occurred on roads around Stockholm in wet weather conditions. The accidents drew attention since they occurred on road sections where problems with polishing weren’t expected. The road was perceived as slippery at the time of the accidents, according to the reports that came in. To investigate the cause of this, a visual inspection was carried out and friction measurements were carried out, which indicated low friction.

The problem of roads becoming polished is a well known issue that is addressed in risk areas such as roundabouts, tight curves or traffic lights. That is, where acceleration and braking or tight turns of the tyres will rub the pavement surface. However, more recently, polished roads have appeared in unexpected places.

The study is funded by the Swedish Transport Administration and the Swedish Construction Industry Development Fund (SBUF). Photo: Hejdlösa Bilder/VTI

“The situation is serious given the high standards for friction set by the Swedish Transport Administration and the fact that the majority of the roads under investigation have sections with poorer friction ratings. In our pilot study, we have started by highlighting some possible causes,” says Henrik Bjurström, project manager and researcher at VTI.

Due to the use of studded tyres during the winter, road surfaces used to be roughened up. This roughening has now been reduced or disappeared entirely. Researchers have measured the friction on a test section of the E4/E20 at Kungens kurva and on a number of access roads around Stockholm. The measurements confirm that the problem of low friction is no longer limited to single road sections but is found on most of the roads examined around Stockholm.

In addition to the fact that the expected roughening has not occurred during the winter period, polishing has continued during the summer period reducing friction even further. Additionally, the effects of friction-enhancing maintenance have only been effective for a short period of time before the friction values have dropped again.

One of the factors identified by the researchers is the new type of studded tyres with less aggressive studs that have reduced wear and roughening by up to 20 percent. Similarly, the use of studded tyres in Stockholm has decreased by 20 percentage points over the past ten years. Instead, friction tires, winter tyres without studs made for Nordic winter driving conditions, are now being used more and more. These friction tyres polish the road surface even more than summer tyres. The increase in traffic and the increased share of heavier and more powerful vehicles are additional causes of the polishing phenomenon.

“We hope to be able to conduct a follow-up project where, with the help of VTI’s test road machine, we can produce better quantitative results and more in-depth studies on the causes of polishing. We will then be able to get closer to the answer as to which factors affect the phenomenon and by how much,” says Henrik Bjurström.

One problem area is the road surface itself. Porphyry and quartzite are two durable stone materials that have been used for a long time for pavement surfaces in Sweden to minimize the damage caused by studded tyres. As less aggressive studs are now being developed and more people are also choosing to use stud-free tyres, it may be necessary to rethink the choice of stone material, according to the researchers. Low wear must be weighed against sufficiently high friction as these parameters are often in opposition to each other.

“Porphyry is the most durable type of stone we have, and the new type of studs don’t roughen the surface. Instead, the stones are polished, becoming smooth and slippery.

Finland has developed tests for studded tyres to be launched on the market that show how much wear they cause on the road. A continuation study could include a comparison with the conditions in Finland and Norway, countries that also use studded tyres and with similar climate conditions as Sweden,” says Henrik Bjurström.

The pilot study is authored by Henrik Bjurström and Roger Nilsson, specialist in road technology at Skanska. The study is funded by the Swedish Transport Administration and the Swedish Construction Industry Development Fund (SBUF).

Text: Gunilla Rech/VTI
Translated by: CBG Konsult & Information AB

Henrik Bjurstörm
VTI, Swedish National Road and Transport Research Institute

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