Capacity of shared left-turn lanes : a simplified approach.

This paper develops a simple and readily usable formula for estimating the capacity of a shared left-turn lane. It also derives a left-turn blockage factor for use in the formula. The capacity of a through lane at a signalised intersection is reduced by factors that reflect (a) opposing left turns or (b) the blockage effect of left turns on through traffic in the same lane. The capacity of a shared lane represents the minimum of these computations. Thus, the capacity of a shared lane is reduced by the number of left turns using the lane, as well as the traffic volumes in the opposing direction. Opposing left turns reduce the capacity of both shared left-turn lanes and through lanes. When there are five or more left turns per cycle in a shared lane, they preempt that lane for all practical purposes. Short signal cycles appear preferable where shared left-turn lanes dominate.