Roundabouts are introduced at intersections to improve traffic flow and enhance safety. Nevertheless, studies and field observations demonstrated that roundabouts, due to their special design, could significantly affect the efficiency of the overall traffic network, especially in the case of increased traffic volumes. For this reason, researchers and practitioners have conducted several studies to alleviate the negative impact of saturated traffic. In these studies, different characteristics of traffic flow and roundabout topologies are analyzed to show their impact on the overall performance. In this paper, we present two simulation studies to investigate the effect of driving speed on the performance of roundabouts with different geometrical characteristics. The results from the two case studies indicate that speed control and the distribution of traffic volumes on the arms of the roundabout are two important factors that affect the performance of roundabouts. Moreover, the results also show that the driving speed factor correlates with roundabouts' geometrical characteristics. Further, the individual driving behavior plays a major role in the performance of roundabouts.

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