Interactive Decision Support System for Predicting Flashing Yellow Arrow Left-Turn Mode by Time of Day

Accommodation of left turns at signalized intersections has been a challenge for traffic engineers seeking a balance between two conflicting goals: capacity and safety. The use of a four-section display for the left-turn lane only with a flashing yellow arrow indication for permissive left turns has been deemed the new standard for signalization. With the advent of this new signal configuration, the opportunity to take the protected–permitted left-turn mode to a new level of operation has been presented. Although numerous studies have developed warrants and guidelines for selecting types of left-turn control, no clear or uniform standards for the implementation of a variable left-turn mode that changes by time of day are currently available. This study developed an interactive decision support system for evaluating left-turn phasing alternatives by time of day by the use of a custom-design approach. The resultant model predicts the number of left turns during the permitted phase under different intersection conditions and assesses the operational and safety aspects warranting a permitted left-turn phase on the basis of specific criteria and thresholds; comparison of the amount of permitted green time given throughout the hour with the number of permitted left turns determines whether the opposing traffic flow is operating near or at saturation. The guidelines that were developed will provide traffic engineers with the tools that they need to use the efficiency of the permitted left-turn phase. Furthermore, the decision support system that was developed will assist traffic management centers with the identification of intersections requiring attention to or modification of the left-turn mode.