Critical movement analysis for shared left turn lanes.

This paper analyses critical movements for left turn lanes that are shared by through traffic. It shows that the critical conflict volume along a given approach is the sum of the left turns, the opposing traffic, and the blocked proportion of the through traffic in the shared lane. A set of impedance or blockage factors is derived as a function of the left turns per cycle. When two left turns per cycle occur, 60 percent of the through vehicles in the lane are blocked.When five left turns per cycle exist, 80 percent of the through vehicles in the shared lane are blocked. Thus, for all practical purposes, five or more left turns per cycle will pre-empt the shared lane.From this it follows that short signal cycles are desirable where shared left turn lanes predominate. The paper addresses shared lanes in both single and multilane approaches. It contains guidelines for computation of critical movement volumes in practice.