Analysis of speed characteristics of urban expressways under rainy conditions

Rainfall has a significant impact on the operation of urban expressways. Based on data collected by loop detectors, typical segments with two, three and four lanes in one direction are selected from the South-North Expressway in Shanghai. Rain intensity is classified into light, moderate, heavy and torrential in this study. Firstly, an exponential speed-density model is used to describe the relationship of speed and density under both clear weather and rainfall. Based on the results of model fitting, the effects of rainfall on free flow speed and operating speed are studied. Results show that free flow speed decreases most on segments with two lanes, with 0.3-5%, 5.1-7.4%, 4.6-8.2% and 9.8-13.3% reductions under light, moderate, heavy and torrential rain, respectively. This suggests that drivers' sight and judgement are easy to be affected on segments with only two lanes. In addition, a reduction factor is introduced to study the influence of rainfall on operating speed, and traffic flow is divided into congested and uncongested flow with the optimum density. The reduction of operating speed under congested conditions is larger than that under uncongested conditions, especially for segments with three and four lanes. So it is essential to make sure the traffic operation of segments with more than two lanes in each direction of urban expressways on rainy days.