Guidelines for the Use of Protected/Permissive Left-Turn Phasing

The objective of this study was to develop guidelines that may be used to aid the traffic engineer in deciding whether protected/permissive (permissive) phasing is appropriate to use at a given location. An inventory identified 58 intersections in Kentucky where permissive phasing was used. Characteristics of these intersections were related to accident data to determine the effect permissive phasing has had on p.eft-turn and total accidents. Left-turn accidents increased dramatically when permissive phasing replaced protected-only (exclusive) phasing and when it was used at approaches where the speed limit was over 45 mph. There were no substantial changes in left-turn and total accidents when permissive phasing was part of the original signal installation or was the first left-turn signal at an existing signal and where the speed limit was 45 mph or less. When a left-turn signal is warranted, permissive phasing should be used because of the reduced delay compared to exclusive phasing, unless the intersection has certain characteristics for which permissive phasing would create an increased accident potential. A list of conditions that would preclude the use of permissive phasing is presented.