Effects of unbalanced approach volumes on roundabout operations

Modern roundabouts are increasingly popular in the United States due to their advantages in improving safety and reducing delay at intersections. A roundabout is an intersection in which traffic enters the intersection and travels in a circular counterclockwise motion around a center island. In a roundabout, priority is given to vehicles traveling in the circular roadway, and incoming traffic at the roundabout approach must yield to the circulating traffic. If there is an unbalance in approach volume, the approaches with higher input volumes may dominate the circulating flow preventing vehicles the other approaches to enter the roundabout. This may consequently cause excessive delay at certain approaches and affects the overall performance of the roundabout. There are insufficient studies where the effects of unbalanced approach volumes at a roundabout are analyzed. This research analyzed the average control delay and level of service of a typical four-leg, two-lane modern roundabout with different combinations of approach volume for three different unbalanced scenarios. The microscopic traffic simulation software, VISSIM, was used to simulate the three unbalanced scenarios under different approach volume combinations that ranged from 400 pc/h to 1600 pc/h. The roundabout model’s parameters were calibrated using data collected from a roundabout in Olathe, Kansas that has the similar geometric attributes as the roundabout analyzed. The measures used to analyze the roundabout operational performance were average control delay and level of service. The results are presented in three charts for the three different unbalanced scenarios under different volume combinations. These charts serve as a reference guide for engineers to determine if a roundabout is feasible for implementation at an intersection under a given traffic demand