Traffic safety effects of narrow 2+1 roads with median barrier in Sweden

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Deaths on rural roads are a serious road safety problem. Due to the risks imposed by high speeds, multi-functionality, lower infrastructure safety and mix of different road users, rural roads are often dangerous roads with relatively high risk levels compared to motorways. In Sweden, a special road safety concern during the 1990’s was the large amount of fatal and serious crashes on rural 13 m wide 2-lane roads. The solution proved to be a redesign to a 2+1 road usually with a median barrier. Starting 2009, this solution was also applied on rural roads with road width of about 9 – 10 meters. A 2+1 road with median barrier has a continuous three-lane cross section with alternating passing lanes to allow defined sections of overtaking, the two directions of travel separated by a flush divider with a median barrier. Comparing 13 meter wide 2+1 roads with narrow 2+1 roads (9 m), the main difference is the length and frequency of passing lanes. For narrow 2+1 roads, the share of passing lanes varies between 15 – 30% compared to about 40% for the 13-meter roads.

The main objective of this study is to evaluate the traffic safety effects of these narrow 2+1 roads. A before and after study with control group is performed based on crash statistics from the Swedish crash data base STRADA. Using a control group, the results have been adjusted for the general road safety trend and changes of traffic volumes. In addition, a limited Empirical Bayes study was done to adjust for regression to the mean.

Results from the before and after study show a number of significant effects; the total number of fatalities and seriously injured decreased by 50 % and the total number of personal injury crashes decreased by 21%. The severity consequence (the rate of the number of killed and seriously injured divided by the number of personal injury crashes) decreased by 38%. Looking only at links (excluding intersections), the number of fatalities and seriously injured decreased by 63% and the personal injury crashes by 28%. Correcting for regression to the mean gave very similar results. It should be noted that the after period is still short and a continued follow up will be done during 2015. For almost all of the included road sections, the speed limit was also raised from 90 km/h to 100 km/h when the road was rebuilt to 2+1. No difference in efficiency compared to earlier evaluations of traditional 2 +1 roads with 100 km/h and 40% passing lanes can be observed.

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