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

Abstract

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.

Keywords

• Traffic engineering
• Intersection
• Left turn lane
• Offset
• Safety
• Operation