Study Details

Study Title: Left-turn Phase: Permissive, Protected, or Both?

Authors: Li Chen, Cynthia Chen, and Reid Ewing

Publication Date:JAN, 2012

Abstract: The practice of left-turn phasing selection (permissive, protected-only, or both) greatly varies from one locality to another. The literature evidence on this issue is equally mixed and insufficient. In this study, we evaluate the safety impacts of changing left-turn signal phasing from permissive to protected/permissive or protected-only at 68 intersections in New York City using a rigorous quasi-experimental design accompanied with regression modeling. Changes in police reported crashes including total crashes, multiple-vehicle crashes, and left-turn crashes, were compared between before period and after period for treatment group and comparison group by means of negative binomial regression using a Generalized Estimating Equations (GEE) technique. Confounding factors such as the built environment characteristics that were not controlled in comparison group selection are accounted for by this approach. The change of permissive left-turn signal phasing to protected/permissive or protected-only signal phasing does not result in a significant reduction in intersection crashes. Though the protected-only signal phasing does reduce the left-turn crashes, this reduction was offset by the possible increase in over-taking crashes. These results suggest that left-turn phasing should not be treated as a universal solution that is always better than the permissive control for left-turn vehicles. The selection and implementation of left-turn signal phasing need to done carefully, considering the trade-offs between safety and delay, and many other factors such as geometry, traffic flows and operations.

Study Citation: Chen, L. C. Chen, and R. Ewing. "Left-turn Phase: Permissive, Protected, or Both?" Presented at the 91st Annual Meeting of the Transportation Research Board, January 22-26, Washington, DC, 2012.

Study Report: Download the Study Report Document


CMFs Associated With This Study

Category: Intersection traffic control

Countermeasure: Change permissive left-turn phasing to protected only

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.45552 StarsAllAllNot SpecifiedUrban
0.44562 StarsAngle,Head on,Left turn,Rear end,Rear to rear,Right turn,SideswipeAllNot SpecifiedUrban
0.23772 StarsLeft turnAllNot SpecifiedUrban
0.49512 StarsRear endAllNot SpecifiedUrban
0.36642 StarsHead on,SideswipeAllNot SpecifiedUrban

Countermeasure: Change permissive left-turn phasing to protected only or protected/permissive

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.58422 StarsAllAllNot SpecifiedUrban
0.62382 StarsAngle,Head on,Left turn,Rear end,Rear to rear,Right turn,SideswipeAllNot SpecifiedUrban
0.7302 StarsLeft turnAllNot SpecifiedUrban
0.59412 StarsRear endAllNot SpecifiedUrban
0.36642 StarsHead on,SideswipeAllNot SpecifiedUrban

Countermeasure: Change permissive left-turn phasing to protected/permissive

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
0.67332 StarsAllAllNot SpecifiedUrban
0.68322 StarsAngle,Head on,Left turn,Rear end,Rear to rear,Right turn,SideswipeAllNot SpecifiedUrban
0.83172 StarsLeft turnAllNot SpecifiedUrban
0.63372 StarsRear endAllNot SpecifiedUrban
0.37632 StarsHead on,SideswipeAllNot SpecifiedUrban