Study Details

Study Title: Safety Impacts of Using Short Left-Turn Lanes at Unsignalized Median Openings

Authors: Chen et al.

Publication Date:JAN, 2014

Abstract: The AASHTO Greenbook specifically encourages the use of left-turn lanes at median openings on divided roadways to eliminate stopping in through-traffic lanes. However, in urban areas, it is often impractical to provide the Greenbook recommended lengths for median left-turn lanes when the available length between two adjacent openings is inadequate, which is particularly evident in the case of heavy left-turn volumes. Thus, left-turn lanes shorter than the Greenbook lengths (referred to as "short left-turn lane" in this paper) are in wide use on urban divided roadways. The objective of this study was to investigate the safety performance of short left-turn lanes at unsignalized median openings. To this end, six years of crash data were collected from fifty-two median left turn lanes in Houston, Texas, which included forty short lanes and twelve lanes that adhered to the Greenbook recommendations. A Poisson regression model was developed to relate traffic and geometric attributes to the total count of rear-end, sideswipe, and object-motor vehicle crashes at a left-turn lane. Crash modification factors (CMFs) were calculated for future applications in projecting the crash frequency, given a specific change of the lane length. It was statistically evidenced that the difference between actual lane length and the Greenbook recommended length had significant effects on the crash frequency. However, the increase of crash frequency due to short left-turn lanes might be acceptable in some cases, in which engineers also need to account for traffic, economic, and social impacts in determining whether a short left-turn lane is appropriate.

Study Citation: Chen, X., Y. Qi, and Y. Lu. "Safety Impacts of Using Short Left-Turn Lanes at Unsignalized Median Openings". Presented at the 93rd Annual Meeting of the Transportation Research Board, Paper No. 14-0549, Washington, D.C., (2014)

CMFs Associated With This Study

Category: Access management

Countermeasure: Shorten left turn lane by X percent at unsignalized median opening

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
CMF EquationCRF Equation3 StarsFixed object,Rear end,SideswipeAllNot specifiedUrban