Review of United States research and guidelines on left turn lane offset:Unsignalized intersections and signalized intersections with permitted left turns

The purposes of this paper are to better understand the design and impact of left turn lane offset and provide state transportation agencies recommendations on best practices and alternative design options to address left turn sight line obstruction issues.Research studies and existing guidelines on left turn lane offset were reviewed and summarized to offer insights in driver sight distance and behavior,intersection safety and operations,as well as design elements related to left turn lane offset.Studies showed that at both unsignalized intersections and signalized intersections with permitted left turns,obstructed sight line could cause higher possibilities of collisions between left turning vehicles and oncoming vehicles from the opposing direction.Existing evaluations of left turn lane offset,with data from multiple states in the United States,reported that positive left turn lane offsets were beneficial in terms of safety and operations.Most agency guidelines provided very limited discussions about left turn lane offset.This review demonstrated that positive left turn lane offsets are beneficial to intersection safety and operations.Based on the review of research and guidelines,recommendations were provided for transportation agencies for left turn lane offset implementation.A discussion on new left turn design concepts was also included to point out directions for future research and practice.

Safety effectiveness of roadway conversion with a two way left turn lane

In urban or suburban areas with a large number of access points, four-lane undivided highways are prone to crashes due to left-turning and through movements in a single lane. Many studies recommended expensive countermeasures like conversion from undivided to divided road with physical separation. One inexpensive alternative is reconfiguring the existing roadways by either increasing or decreasing the number of lanes. This study investigated the safety impact of converting four lane undivided roadways （4U） to five lane undivided roadways （ST） with a two way left turn lane （TIkrLTL）. This study used Empirical Bayes method to determine the safety impact of this inexpensive countermeasure. In this study, data from eight sites from Louisiana were collected for investigation, and site- specific crash modification factor （CMF） values were calculated. Although ST is usually not preferable due to its exposure of higher number of crashes in the existing literature, the findings of the current study indicated a positive safety impact. The benefit-cost ratio of this conversion ranges from 97 to 379. The current findings indicate that conversion of 4U to 5T is a feasible inexpensive solution for urban roadways with lower volume and a limited number of driveways.