Modeling the Temporal Priority Reversal Phenomenon at Roundabouts
AuthorAhiamadi, Samuel Kofi
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Roundabouts are a popular intersection control in the United States with over 2000 constructed in more than 45 states after their introduction in 1990. They have become a preferred choice over stop-controlled and signalized alternatives at low to medium traffic volumes for some jurisdictions. The number of roundabouts in the U.S is progressively increasing annually because of the safety and operational performances obtained with their installation. Procedures for roundabout analyses and evaluation in the 2010 U.S. Highway Capacity Manual are based on the assumption of absolute priority to circulating traffic: meaning entering drivers have to seek for gaps of sufficient size in order to merge with the circulating traffic. However observations at roundabouts operating at or near capacity show periods of priority reversal or shared priority between entering and circulating vehicles. Other studies of the priority reversal phenomenon have reported significant influence on the capacity, delay and queue length at roundabouts. To study the effects of priority reversal on the performance of roundabouts in the U.S., a roundabout in Fernley, Nevada, operating at capacity during the P.M. peak period was observed. The driver behavior characteristics were extracted from video recordings and the geometric parameters were obtained from the site. Based on the observed driver behaviors and traffic flow characteristics, four scenarios were modeled in the micro-simulation software VISSIM. Results obtained after multiple simulation runs showed improvements in the performance of roundabouts as the periods of priority reversal increased: delay reduced by 8-16 percent and queue length reduced by 10-20 percent for every 10 percent increase in "reversed priority periods". It was also shown that, as the priority reversal period increased, the influence of the size of the inscribed diameter decreased even though an improvement in performance was observed. When the intersection experienced a traffic volume increased, the performance of the roundabout decreased until it reached a threshold level where it appeared to remain constant. These modeling results were incorporated into the Analytical Hierarchy Approach, a multi-criteria decision based application to develop a tool that can be used to compare roundabouts to other intersection control options. This tool revealed a potential to enable better comparison devoid of biases and decision makers' preferences.