Capacity reduction and its seasonal change at a bottleneck on an urban expressway during sunset

This study investigates the capacity changes at several bottlenecks on an urban expressway during sunset. In previous studies, an increase in the road capacity during sunrise has been observed. This result implies that a significant change in capacity can also be observed during sunset when higher demand exists. The traffic flow data collected by using detectors installed on the Hanshin Expressway, which is an urban expressway in Japan, is examined. A capacity reduction, corresponding to the change in the sunset time, is observed. The relationship between the traffic volume and density during sunset is analysed to estimate the amount of capacity reduction. Applying Drake's formula, it is estimated to be about 5%. During the period of reduction, the speed of vehicles decreases, whereas the distance between them remains constant. This implies that the drivers maybe reducing their speed when the brightness of the surroundings decreases.