Use of Directional Median Openings on Urban Roadways
Yi Qi, Xiaoming Chen, Yubian Wang, Guanqi Liu, and Yan Lu, May 2014
Over the past decades, many states and local transportation agencies have installed directional median openings on divided roadways to improve arterial safety and operational performance. A directional opening is normally used to restrict crossing and left-turn movements from minor streets to help avoid potential conflicts. A series of potential benefits may be achieved by installing directional-median openings, including reduced crash rates, increased traffic capacity, and better operational performance. However, the benefits of directional median openings depend largely on proper implementation and on various factors, including geometric, traffic control, environmental conditions, and the type and placement of the downstream U-turn provisions. The goal of this research is to investigate the safety impacts of installing directional openings on median-divided urban roadways. To achieve this goal, the research : 1) synthesized existing related research; 2) compared the safety performance of directional median openings and full median openings, at subject opening locations and downstream U-turn locations; and 3) analyzed the contributing factors to the crashes occurred at the downstream U-turn locations of a directional median opening.
The studies led to a number of findings. Some of the highlighted findings include: 1) Converting a full median opening to directional median opening will reduce the crash frequency at the subject opening location. Although directional median opening might increase the crash frequencies at downstream U-turn locations, the total crashes at subject openings and downstream U-turn locations are still lower than that at full median openings.; 2) The total numbers of crashes at downstream U-turn locations of directional median openings were significantly affected by downstream U-turn volume, downstream left-turn volume, and distance to downstream U-turn opening. Higher downstream U-turn volume and downstream left-turn volume would result in more crashes at downstream U-turn locations. The closer the downstream U-turn location to the subject opening, the more crashes at downstream U-turn location. These findings indicates that, since converting a full median opening to a directional median opening will generate more U-turns at downstream opening of directional opening, the selection of U-turn location is critical for the safety performance of directional openings. Diverted left-turn traffic should not be allowed to make U-turns at closely spaced openings that already have significant U-turns or left-turn volumes.
Keywords: Directional Median Opening, Full Median Opening, Downstream U-turn Location, Poisson Regression Model
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