Development of Guidelines for Permitted Left-Turn Phasing Using Flashing Yellow Arrows

Principal Investigator(s):

Gary Davis, Professor, Civil, Environmental and Geo-Engineering


  • John Hourdos, Former Research Associate Professor, Civil, Environmental and Geo-Engineering

Project summary:

The objective of this project was to develop guidelines for time-of-day use of permitted left-turn phasing, which could then be implemented using flashing yellow arrows (FYA). This required determining how the risk for left-turn crashes varied as traffic-flow conditions varied during the course of a representative day. This was accomplished by developing statistical models, which expressed the risk of occurrence of a left-turn crash during a given hour as a function of the left-turn demand, the opposing traffic volume, and a classification of the approach with respect to the opposing traffic speed limit, the type of left-turn protection, and whether opposing left-turn traffic could obstruct sight distance. The models were embedded in a spreadsheet tool that will allow operations personnel to enter, for a candidate intersection approach, existing turning movement counts and a classification of the approach with respect to speed limit, turn protection, and sight distance issues and receive a prediction of how the risk of left-turn crash occurrence varies throughout the day relative to a user-specified reference condition.

Project details: