An Analysis of the impact of Geometry Design to the safety of Interchange Ramps of Urban Expressways

interchange ramps of Urban expressways,serving a large number of turning flow and with a low design standard,are accident-prone.In order to evaluate the safety of interchange ramps,accidents data and traffic flow data in recent 3 years at Puxi Expressway in Shanghai are collected.Based on vehicle behavior and interactive features of vehicles on the ramps,interchange ramps are divided into 5 study units:exit segment,connecting segment,entrance segment, left-turn ramp and right-turn ramp.For each unit,a Negative Binomial Model is developed to analyze the relationship a-mong road geometry design features,their composite effect and traffic flow,and their impacts on safety.The results show that the safety of exit and entrance segments is closely related to geometry design.Compared with right-turn ramps, the left-turn roundabout ramps are more likely to be affected by geometry design.With a higher traffic flow or longer road segment,the risk of traffic accidents is higher.However,longer exit segments are safer.The ramps that have straight line at the exit,or have long-straight downhill section at the entrance,or have a large design difference between entrance and main segment,are less safe.There are more traffic accidents taking place on left-turn roundabout ramps with small radius curve,especially when the curve is located on the exit of ramps.