Simulation Model for Exclusive Left-Turn Phasing

At the present time, no numerical guidelines or warrants are available for the determination of an exclusive phase for a designated left-turn lane at a signalized intersection. Based on storage requirements for a left-turn lane operating as a permitted movement, a quantitative guideline was developed to determine the need for separate phase control. Queue lengths were derived from a simulation model that represents the operation of a designated left-turn lane with opposing traffic movements. In this model, storage distance is related to left-turn and opposing-traffic volumes, cycle length, and green time. For specified left-turn storage lengths, limiting left-turn volumes were selected for a range of opposing traffic levels. No exclusive phase is stipulated for left-turn movements below the critical limits, while a protected or permitted-protected phase is specified when these limits are exceeded. This criterion is predicated on the need for unblocked lanes on an intersection approach with signalized control. Design charts were generated to facilitate the selection of permitted or protected signal control for a designated left-turn lane.