Safety assessment of passing relief lanes using microsimulation-based conflicts analysis.

The paper target is to analyze the influence of passing lane section and merging area lengths, which represent a critical part in the geometric design of passing lanes. The lack of observed data and the still limited implementation of short passing 2+1 lane in Poland do not allow to perform a reliable performance-based design of their length to balance context and economical restrains with operational and safety performances. Besides, defining the optimum geometric design from observed data is a reactive approach which require detailed observations and evaluations of the safety and operational performances for different geometric configurations. The present research work aims to fill the gap by performing an evaluation of optimal length, based on microsimulation analysis. Particular attention was paid to the calibration and validation of microsimulation models using real data of traffic characteristics and crash history. The assessment was conducted using simulation models in VISSIM and Surrogate Safety Assessment Model (SSAM) software and comparison based on simulated results at different lengths of passing lane, merging area in a wide range of AADT values. The results suggested not only that conflicts can be a surrogate measure of safety but also that the influence of geometric parameter, such as the length of the additional lane, plays a fundamental role in the safety performance of the 2+1 short passing lane.