Safety-Related Guidelines for Time-of-Day Changes in Left-Turn Phasing

Abstract

The goal of this research was to develop safety-related guidelines for time-of-day use of permissive left-turn phasing that could be implemented with flashing yellow arrows. This effort required describing the variation in left-turn crash risk throughout a representative day and was accomplished by developing statistical models that related the hourly risk on an approach to left-turn demand, opposing traffic volume, and a classification of the approach with respect to operational and geometric features. The models were then embedded in a spreadsheet tool that allowed signal operators to enter—for a candidate intersection approach—a turning movement count and a classification of the approach according to speed limit, left-turn protection, and sight distance availability. The user receives a prediction of how the risk of left-turn crash occurrence varies throughout the day, relative to a user-specified reference condition.