The assessment of road safety for passing relief lanes using microsimulation and traffic conflict analysis

Most crash prediction models are statistically-based prediction methods which require significant efforts in crash data collection and may not be applied in particular traffic environments due to the limitation of data sources. Traffic conflict studies have been proposed to mitigate this issue using short time survey to count traffic conflicts which are assumed as a surrogate measure of safety. Unfortunately, they are again field-based studies which can be carried out after the implementation of the treatment. Nowadays, simulation tools can be utilized in traffic conflict studies. Traffic conflicts can be detected through SSAM where vehicle trajectories from micro-simulation software (e.g. VISSIM) are post-processed. In this framework the present research work deals with passing relief lanes safety performance. According to international experiences, such sections provide an improvement of traffic and safety performance in comparison to two-lane rural roads. Indeed, to achieve safety benefits, the length of a passing lane section should not be shorter than the recommended minimum value. In accordance with technical standards of different countries, those lengths vary from 0.5 km (Poland) to 1.2 km (Austria). The reasons behind the application of shorter lanes in Poland include the high density of intersections and budget constraints. Short passing sections are a cause of concerns because they can lead to hazardous maneuvers and may increase the number of conflicts in the merging area. The lack of wide implementation of short passing relief lanes does not allow the performance of a reliable traffic safety analysis due to the lack of observed data. The present research paper aims to fill this gap by performing a safety evaluation of 2+1 cross-sections with short passing relief lanes, where the discharge of platoons is distinctly reduced. The proposed models provide a simple approach in determining the boundary conditions regarding safety to determine the minimum required length of a passing relief lane.