Turner-Fairbank Highway Research Center

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Safety Effectiveness of Intersection Left-and Right-Turn Lanes

Product Type

Research Report


Douglas W. Harwood, Karin M. Bauer, Ingrid B. Potts, Darren J. Torbic, Karen R. Richard, Emilia R. Kohlman Rabbani, Ezra Hauer, Lily Elefteriadou, and Michael S. Griffith


This paper presents the results of research that performed a well-designed before-after evaluation of the safety effects of providing left- and right-turn lanes for at-grade intersections. Geometric design, traffic control, traffic volume, and traffic accident data were gathered for a total of 280 improved intersections, as well as 300 similar intersections that were not improved during the study period. The types of improvement projects evaluated included installation of added left-turn lanes, added right-turn lanes, and extension of the length of existing left- or right-turn lanes. An observational before-after evaluation of these projects was performed using several alternative evaluation approaches. Three contrasting approaches to before-after evaluation were used: the yoked comparison or matched-pair approach, the comparison group approach, and the Empirical Bayes approach. The research not only evaluated the safety effectiveness of left- and right-turn lane improvements, but also compared the performance of these three alternative approaches in making such evaluations. The research developed quantitative safety effectiveness measures for installation design improvements involving added left-turn lanes and added right-turn lanes. The research concluded that the Empirical Bayes method provided the most accurate and reliable results. Further use of this method is recommended.

Link To Research Report

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Traffic Safety
At-Grade intersection
Left-turn lanes
Right-turn lanes
Before-after evaluation
Empirical Bayes method

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