Lengths of Turn Lanes on Intersection Approaches

This paper examines the lengths of turn lanes when a single lane approaches a signalized intersection and is divided into three lanes: left-turn, through, and right-turn. These three lanes help to increase the capacity of the intersection by allowing more vehicles to enter the intersection during the green phase. The question is, What is the appropriate length of each turn lane? The vehicle queue pattern at the entrance to the turn lanes is examined first. The following conditions prohibit an arriving vehicle from entering the desired lane: (a) lane overflow (the lane is too short to accommodate all turning movements) and (b) lane entrance blockage (the overflow of the vehicles from other lanes blocks the entrance of the lane, even though space is available in that lane). A set of formulas to compute the probabilities of these events is developed. The lengths are calculated so that the probability that a lane does not overflow and the entrance of the lane is not blocked is greater than a threshold value. Tables of the recommended lengths are prepared for different turn volumes and threshold probabilities. These lengths are found to be shorter than what is traditionally recommended. The presence of three lanes (left-turn, through, and right-turn) allows arriving vehicles to spread among different lanes, and thus the chances of lane overflow and blockage are reduced. This is noticeable when the movements among the three directions are nearly even.