Left-turn phasing decisions utilizing simulated traffic conflicts and historical crashes

A fundamental objective of traffic signal operations is the development of phasing plans that reduce delays while maintaining a high level of safety. One issue of concern is the treatment of left-turn phasing, which can operate as a protected movement, a permitted movement yielding to conflicting traffic, or a combination protected-permitted movement. Protected-only movements can improve safety of the turning movement, but they can also increase delays and congestion at intersections. Most states use criteria for left-turn phasing selection based on a threshold crash values and do not account for traffic volumes or intersection features that may influence crash frequency. This research leverages conflict points as an indicator of potential safety estimation to assist in the selection of the left-turn phasing and relates them to historical crash records. Prediction models of potential conflicts were developed through microsimulation for 200 existing intersections; hourly volume data resulted in approximately 2,300 hours of observations. The number of left-turn-related conflicts was obtained through SSAM and related to the number of crashes at each intersection. The proposed models offer a simple but realistic approach for determining the boundary conditions that influence safety when left-turn decisions are required. The models can be used to develop nomographs, which practicing traffic engineers can use for left-turn phasing decisions.