Safety Effects of Using Short Left-Turn Lanes at Unsignalized Median Openings

The AASHTO Green Book specifically encourages the use of left-turn lanes at median openings on divided roadways to eliminate stopping in through-traffic lanes. However, the lengths for median left-turn lanes recommended by the Green Book are often impractical in urban areas, where the available distance between two adjacent openings is inadequate, and particularly where the volume of left-turning traffic is heavy. Thus, left-turn lanes shorter than the lengths recommended by the Green Book (referred to as short left-turn lanes) 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, 6 years of crash data were collected in Houston, Texas, from 52 median left-turn lanes (40 short lanes and 12 lanes with lengths in compliance with Green Book recommendations). A Poisson regression model was developed to relate traffic and geometric attributes to the total number of rear-end, sideswipe, and object–motor vehicle crashes in a left-turn lane. Crash modification factors were calculated for applications to project the crash frequency for a specific change in lane length. Statistical evidence indicates that the difference between actual lane length and Green Book–recommended length has significant effects on crash frequency. However, the increase in crash frequency as a result of short left-turn lanes might be acceptable in cases where engineers must account for traffic and economic and social effects in determining whether a short left-turn lane is appropriate.