Influence Mechanism of Rainfall on Traffic State of Urban Road Network
It is a common perception that urban roads are prone to experience traffic congestions in rainy days, but is it true that rainfall will inevitably cause traffic congestion? This paper investigates this mechanism from the theoretical view and actual data analysis. It was found that drivers tend to drive conservatively in rainy days, which increases the saturation headway. Based on this hypothesis, a signal-influenced urban road traffic flow model is established, and the changes in average travel speed under different rainfall scenarios are analyzed theoretically. The saturation headway under each rainfall level obtained by using video data statistics verifies the hypothesis that rainfall increases the saturation headway. Simplifying the data of the main road network in Xiaoshan District, Hangzhou, numerical simulations were conducted and the proposed model was verified through VISSIM road network simulation. The results show that the effect of rainfall on the average speed of the road network is negative. When the traffic demand is small, the negative impact of rainfall is almost negligible; as the rainfall level increases, the road network with high traffic demand is more likely to be oversaturated, and the average travel speed of the road network will experience a steep drop and the traffic condition deteriorates rapidly. Taking the maximum demand of the road network as an example, compared with the non-rainfall weather, the average speed of the road network decreases by 8.63% in the light rainfall scenario, 16.51% in the medium rainfall, 23.43% in the heavy rainfall, and 24.94% in the rainstorm. The VISSIM simulation results fit well with the theoretical values of the model, which verifies the effectiveness of the model. Since the traffic demand during the peak hours is much larger than that during the off-peak hours, the rainfall during the peak hours will bring a greater negative impact on the traffic status of the urban road network, which requires more attention from the traffic management.