Studies on Norwegian Road Tunnels II : an analysis on traffic accidents in road tunnels 2001-2006
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The study is based on 797 national road tunnels with a total length of about 778.5 km. Accidents were recorded in 250 of these tunnels during the study period. When including the last 50 meters before the tunnel entrance, there were recorded tunnel related accidents in 299 of the tunnels. The 926 person injury accidents included in this study resulted in 1428 fatalities or personal injuries. A total of 739 of these accidents occurred within the tunnel itself and the remainder at the last 50 meters before the tunnel entrance. The 739 tunnel accidents involved a total of 1130 fatalities or personal injuries. The three predominant tunnel accident types are same direction (rear end collision and lane change) with 43%, single accidents with 35% and head-on accidents with 15%. Single accidents are most common in single tube tunnels, while accidents involving vehicles in the same direction predominates in dual tube tunnels. This study confirms previous study findings, which show that it is the entrance zones that have the highest accident rate and that the accidents rate declines with distance inwards. Overall accident rate for tunnels is 0.12 and 0.10 for tunnels exceeding 500 meters. When zone 1 is included the overall accident rate is 0.13 and 0.12 for tunnels exceeding 500 meters. A study of the correlation between tunnel length and accident rate supports corresponding findings from all previous studies. Findings show that accident rates vary between 0.22 for tunnels shorter than 100 meters and decline down to 0.08 for tunnels exceeding 3000 meters in length. When considering the correlation between traffic volume and accident rate, it appears that the accident rate declines with increasing traffic. This may be related with the fact that tunnels with little traffic are of a lower standard than those with more traffic. The study also shows a clear correlation between horizontal curvature and accident rate. The accident rate declines with increasing radius. The study does not find the same clear correlation between gradient and accident rate. This might be because accidents caused by a gradient might not always take place within the incline. The study also groups the tunnels into four types: single tube, sub-sea, urban dual tube and rural dual tube. The results show a wide variation in accident rate for these tunnel types. Accident costs for all sub-sea tunnels have not been estimated, but for sub-sea tunnels longer than 300 meters accident costs are 0.34 NOK per vehicle kilometer.