Study Details

Study Title: Safety Effectiveness of Protected Only over Permitted/Protected Left Turn Phasing in Louisiana

Authors: Asaduzzaman et al.

Publication Date:JAN, 2021

Abstract: Roadway intersections are high risks area in a roadway network accounting for significant number of traffic fatalities. Of the total fatalities, intersection fatalities account for around 24% each year with left turning vehicles associated to majority of crashes at intersections. To manage the left turning movements, various left turn signal phasing like permitted (P), protected only (PO), and protected/permitted left turn (PPLT) are currently in use. Though conversion to PO left turn phase has shown positive effects, their overall effectiveness has not been well established in Louisiana. The paper evaluated the safety effectiveness of PO over PPLT phase in Louisiana. A total of 42 four legged signalized intersections were selected with 21 intersections (treatment) having PO left turn phase at all the approaches and 21 intersection (control) with PPLT phase at all the approaches. Treatment and control sites were selected to closely match with their geometry, traffic volume, and roadway classification. A cross-sectional study design was undertaken to develop a safety performance function, using Negative Binomial model, and was used to estimate crash modification factor for total and only left turn crashes at various severity levels. The result shows the positive safety benefits of adding PO over PPLT phase especially on reducing fatal and severe crashes for both total and left turn crashes (CMF = 0.567 and 0.309). However, the result for total and left turn crashes was mixed for other severity levels. With positive effect especially on decreasing severe crashes, installation of PO left turn phase clearly helps to meet the goal of state policies to reduce zero deaths from roadway crashes.

Study Citation: Asaduzzaman, M., R. Thapa, and J. Codjoe. "Safety Effectiveness of Protected Only over Permitted/Protected Left Turn Phasing in Louisiana". Presented at the 100th Annual Meeting of the Transportation Research Board, Paper No. 21-00962, Washington, D.C., (2021).


CMFs Associated With This Study

Category: Intersection traffic control

Countermeasure: Protected/permissive to protected-only left turn phasing

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.001-0.13 StarsAllAllNot specifiedUrban
0.56743.33 StarsAllK,A,B,CNot specifiedUrban
1.367-36.73 StarsAllONot specifiedUrban
0.66343 StarsLeft turnAllNot specifiedUrban
0.30969.12 StarsLeft turnK,A,B,CNot specifiedUrban
1.096-9.62 StarsLeft turnONot specifiedUrban