Optimal Speed Limit by Cost Analysis with Effects of Road and Traffic Conditions in Hokkaido

In Hokkaido, Japan's sparsely populated northernmost island, most drivers usually drive over the posted speed limit due to low traffic volume and excellent road conditions. This raises doubts over the sustainability of the existing speed limits. Therefore, this study aims to determine the optimal speed limit by utilizing cost analysis including the effects of traffic signal intensity and traffic congestion, which are the key factors in determining speed limits and have not yet figured in other similar research. In the cost analysis, the components were travel time cost, vehicle operating cost, pollution cost, and accident cost. Initially, the relationships between average speed and each cost component were determined. After that, the relationships between average speed and total costs were verified so that the optimal average speeds were obtained from the minimum total costs. Then, the effects from traffic signal intensity and traffic congestion were achieved simultaneously through regression analysis. Subsequently, the optimal speed limits were obtained. Finally, the results were compared with the 85th percentile speed and speed limits obtained by considering road characteristics from our previous study. Moreover, to determine model reliability, a sensitivity test was applied. In conclusion, it could be implied that the determination of speed limits by cost analysis, including the effects from traffic signal intensity and traffic congestion, is superior to previous techniques because cost analysis includes all major components and effects for determining the speed limit, especially in terms of road safety.