Estimation of unprotected left-turn capacity at signalized intersections

A mathematical model was developed to calculate the unprotected left-turn capacity of a pretimed signalized intersection. The capacity depends primarily on the volume of traffic opposing the left-turn movement and the percentage of the cycle available for this maneuver. Parameters were determined form field studies conducted in several Texas cities. The model was used to estimate the unprotected left-turn capacity for approaches both with and without exclusive turn lanes. Opposing volumes ranged for 200 to 1000 automobiles/h in one, two, or three lanes. Green splits from 30 to 70 percent of the cycle were analyzed. Unprotected left-turn capacities as predicted by the model and by the Highway Capacity Manual were compared. General agreement was found at a 50 percent green split; significant differences existed at other green splits. /Author/