Safety Evaluation Of Signalized Intersections With Left-Turn Waiting Area

In recent years the metropolitans in China have seen the surging installations of the left-turn waiting area (LWA) at the signalized intersections. The design allows the left-turning vehicles to enter the intersection at the onset of the through green phase (of the same approach) and wait for the exclusive left-turn signal at the LWA. The LWA layout can effectively increase the traffic capacity and reduce the probability of stranded and queue overflow of the left-turn vehicles, but no study is conducted yet to assess the safety performance of the signalized intersections with LWA.This paper analyzes the characteristics of left-turning traffic at intersection with LWA firstly then focuses on its safety performance based on traffic conflict technique. After comparing two primary safety indexes:time to collision (TTC) and post-encroachment time (PET), PET is chose as the surrogate to measure the risk of safety hazards. Base on PET values, it divides the traffic conflicts into three levels:potential conflicts, light conflicts and serious conflicts. The paper studies the mechanism of traffic conflict, compares the discrepancy of conflict types between intersections with LWA and without, and develops the severity models to identify the contributing factors for the left-turn conflicts.Results demonstrate that the operation of intersections with LWA with smaller average PET value is safer than that without. Whereas, the frequency of the left and opposite through conflicts is smaller and the severity is lower. However, the percentage of merging conflicts increases at intersection with LWA. According to the regression result, the left-turn volume, driving outside the LWA, running red light, the presence of secondary conflicts, and the rear-end conflicts significantly increase the severities of traffic conflicts at the LWA.The findings serve to provide recommendations to revise the current design standard of the LWA and consequently improve the safety operations of signalized intersections with LWA in China.