GUIDELINES FOR TURN LANES ON TWO-LANE ROADWAYS

Intersections constitute a very small part of rural and urban street/highway systems, yet are implicated in over half of the motor vehicle accidents. Intersection elements which are related to intersection accident rates include geometric layout and traffic controls. This study concentrates on when separate turn lanes should be required and how thy should be designed. The purpose of a separate turning lane is to expedite the movement through traffic, increase intersection capacity, permit the controlled movement of turning traffic, and promote the safety of all traffic. Research that was reviewed for this study provides criteria for providing separate left or right turn lanes at both signalized and unsignalized intersections. Warrants for providing separate turn lanes or the design details used to construct separate turn lanes are not consistent across the country. Conditions at a specific intersection may warrant the use of dimensions that exceed those in this report. It is also possible that local conditions are so restricted that design values less than desirable must be used to install the only feasible improvement. Although great care should be taken when using below recommended values, it frequently can be more beneficial to install a substandard left turn lane than not provide any separate turn lane.