Determining Lengths of Left-Turn Lanes at Signalized Intersections Under Different Left-Turn Signal Schemes

An analytical procedure was developed to determine the lengths of left-turn lanes at signalized intersections. A large body of literature exists about this subject; however, most examine the case when the left-turn and through movements take place during the same signal phase (split phase). Many state departments of transportation base their guidelines on this case also. The general framework was developed for determining the lengths of the left-turn lanes that prevent lane overflow and blockage of the entrance of the left-turn lane by the queued through vehicles. The framework considers many factors: arrival rates and the sequence of left-turn and through vehicles, different signal schemes (the split-phase, permissive-only, protected-only leading, protected-only lagging, and protected-permissive left-turn phases), and intersection capacity. All possible queue patterns (including the leftover from the previous cycle) were identified, and the probabilities of lane blockage and lane overflow were obtained for different combinations of the parameters. The recommended lengths that prevent lane overflow and blockage of more than 95% of the cycles were developed. The lengths suggested by the existing guidelines were useful within certain ranges of combinations of left-turn and through volumes. Suggestions are made for determining the lengths for outside this limited range of combinations. The framework and suggested lengths should be useful for identifying the options of extending lane length or changing the signal scheme to manage lane overflow and blockage and for evaluating the adequacy of the current length.