Effect of tyre tread depth on accident involvement during summer in Finland

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Clifton Daniel
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1 Young Researchers Seminar 2011 Young Researchers Seminar 2011 DTU, Denmark, 8 10 June, 2011 DTU, Denmark, June 8-10, 2011 Effect of tyre tread depth on accident involvement during summer in Finland Riikka Rajamäki VTT Technical Research Centre of Finland

2 Common background (1) Small tyre tread depth increases stopping distance and likelihood of aquaplaning -> Is it possible to see this effect in accident statistics? Tire contact area on 3mm layer of water. Source: Autonrengasliitto ry 2

3 Common background (2) Some studies in 1970 s and 1980 s Tyre tread depth less than 0.8 mm ->more wet road accidents Tread depth of 0.5 mm -> 15 times more accident involvements than with 5 mm tread depth Elvik s meta-analysis based on two studies: Increasing tread depth from less than 2 mm to 2 3 mm cuts accidents by 19%. Increasing tread depth from 2 3 mm to 3 5mm cuts accidents by 9%. 3

4 Background in Finland Winter tyres are compulsory from 1st December to the end of February Studded tyres may be used from 1st November until the first Monday after Easter. -> Most cars are equipped with two sets of tyres The minimum legal tread depth for summer tyres is 1.6 mm Tyre safety campaign every autumn Tyres are considered as a contributing factor in every seventh fatal car accident. 4

5 Method and data Fatal accidents investigated by investigation teams, years Cars and vans in fatal accidents in summer period (May October) minimum tyre tread depth Lots of road and driver data Tyre check data by year and municipality classified by minimum tyre tread depth: less than 1.6 mm (illegal) mm 3 mm 4 mm or more Kilometrage with different tyre tread depths in summer period Official kilometrage statistics and summer period s share of kilometrage Fatal accident distribution on dry and wet road surfaces with different tyre tread depths All drivers Two driver groups separated by driver behaviour Fatal accident involvement rate with different tyre tread depths All drivers Estimate of two driver groups separated by driver behaviour 5

6 Kilometrage and accident participation by tyre tread depth Summer period car and van kilometrage, average for Tyre tread depth, mm Average kilometrage, million km % % % % 4 or more % Total % Cars and vans involved in summer period fatal accidents, total for Tyre tread depth, mm Number of vehicles % % % % 3.5 or more % Unknown 126 8% Total % 6

7 Risk of being involved in a fatal accident 1,4 Involvements in fatal accidents / 100 million vehicle km 1,2 1,0 0,8 0,6 0,4 0,2 1,18 0,57 0,39 0,16 0,16 0,18 Risk of being chiefly responsible participant for a fatal accident Risk of being an other participant of a fatal accident 0,42 0,50 0,23 0,23 0, Total Minimum tyre tread pattern depth, mm 7

8 Accidents on dry and wet roads 100 % Share of participants, % 80 % 60 % 40 % 20 % 0 % Snow or ice Damp or wet Dry Participants causing accident Other participants Minimum tyre tread pattern depth, mm 8

9 Two driver groups on the basis of compliance with traffic rules 680 drivers who during the accident complied with the main traffic rules: having a driving licence, not intoxicated, using a seatbelt, following the speed limit or exceeding it by no more than 10 km/h 680 drivers who during the accident broke some of the main rules: no driving licence, intoxicated, not using a seatbelt or exceeding the speed limit by more than 10 km/h 9

10 Tyre tread depth distribution in two driver groups 100 % Share of tyre tread depths 80 % 60 % 40 % 20 % 0 % 56 % 19 % 13 % 12 % 70 % 67 % 16 % 19 % 9 % 10 % 5 % 4 % 3.5 mm mm mm mm Drivers who broke the main rules Drivers who complied with the main rules Distribution of vehicle kilometrage based on tyre checks 10

11 Risk of being involved in a fatal accident, estimates for two driver groups Drivers who broke the main traffic rules Drivers who complied with the main traffic rules 4 1,5 Relative risk to be involved in a fatal accident 3,5 3 2,5 2 1,5 1 0, Relative risk to be involved in a fatal accident 1 0, Minimum tyre tread pattern depth, mm Minimum tyre tread pattern depth, mm Note that there are only relative risks so that the risk for a tyre tread depth of 3.5 mm or more is 1. 11

12 Accident participation on dry and wet roads for two driver groups 100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 % 0 % Unknow n Snow or ice Damp or wet Dry Drivers w ho broke the main rules Drivers w ho complied w ith the main rules Minimum tyre tread pattern depth, mm 12

13 Conclusion (1) Car s tyre tread depth correlates with the driver s risktaking in Finland. Among cars and vans with illegal tyre tread depth, 70% of drivers drove intoxicated, was speeding, had no driving licence, or was not wearing a seatbelt A vehicle with illegal less than 1.6 mm tyre tread depth is 2.8 times more likely to cause a fatal accident than a vehicle with at least 3.5 mm tyre tread depth. However, this is not mainly due to the tyres. 13

14 Conclusion (2) Tyre tread depth s effect on risk-taking driver s fatal accident risk was not detected Some effect on rule-complying driver s fatal accident risk (~+15 %), if tyre tread depth is illegal Important to link tyre information with other safe driving instructions and traffic control 14

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