SPR 14-481: Permitted Left Turn Guidance

A fundamental objective of traffic signals is the development of signal plans that improve efficiency of operations and 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, a combination protected-permitted movement, or as a split-phase intersection. While protected-only movements and split-phasinge can improve safety, it can also increase the delays and congestion at the intersection. Permitted movements can safely serve traffic when volumes are low, such as during off-peak periods, but may experience safety or capacity problems with high volumes, such as the AM and PM rush hour. Due to the varying traffic demands throughout the day, the choice of left-turn phasing is not always clear. Current policy also prescribes the use of protected-only phasing for certain geometric configurations, such as when three or more opposing through lanes are present or when dual left-turn lanes are used. It is expected that this research will be used to improve flow and reduce delays by creating a more appropriate left-turn phasing guidance. The anticipated benefits are expected to include lower delays for traffic, potential safety improvements, and a more robust left-turn evaluation and accommodation policy. The primary objective of this research is to determine any potential changes to existing practices and policies with regards to permitted left-turn phasing to improve vehicular efficiency and intersection safety. The resulting guidance will aim to determine appropriate use of permitted, protected, and protected-permitted left-turn phasing. 1