Modeling the effects of rainfall intensity on traffic speed, flow, and density relationships for urban roads

The relationships between traffic speed, flow, and density under various rainfall conditions on urban roads in Hong Kong are investigated in this study. The effects of rainfall intensities are examined empirically, particularly their effects on the reduction of key traffic stream parameters such as free-flow speed, speed at capacity, and capacity (or maximum flow) on urban roads. Rainfall intensity has been found to have significant impacts on urban road key traffic stream parameters, but not on traffic jam density. Generalized speed-flow and speed-density functions are also proposed and calibrated, taking into account the effects of varied rainfall intensity. The calibrated generalized functions are validated with an independent data set from the same location. The generalized speed-flow and speed-density functions are useful for assessing the performance of urban roads, particularly in cites with relatively high annual rainfall intensity. It is believed that the findings could provide better insights into the effects of rainfall intensity on urban road traffic environments for modeling their impacts on road users' route choice behaviors.