Date of Award


Degree Type


Degree Name

Doctor of Philosophy in Civil and Environmental Engineering


Civil and Environmental Engineering

First Advisor

Christopher Hunter


Currently, there is no United States guideline on how the additional lane lengths affect roundabout operation. The purpose of this research is to provide an insight on how the use of an additional lane as an approach affects roundabouts. Hence, most transportation professionals refer to studies conducted overseas that do not necessarily translate directly to domestic roundabout design and operation. As interest continues to grow in the deployment of modern roundabouts in the United States, there is the need to provide effective information to professionals on roundabout design and its effect on operations. Because of this, the purpose of this research is to provide insight on how the use of an additional lane on an approach affects roundabout operations.

Using delay as the measure of effectiveness, a hypothetical four-leg, double-lane roundabout with additional lane design at both entry and exit is analyzed. The additional lane lengths are varied at both entry and exit in order to study the effect of different additional lane lengths on roundabout operation. Similar length variations are applied to an existing roundabout with known data after calibration and validation. The research indicated that very long additional lane lengths resulted in higher speeds on the approach, but were not necessarily providing the greatest overall impact in reducing delay through the roundabout. Through the analyses of both hypothetical and existing roundabout models, there are diminishing returns on reduction of overall delay as the additional length increases or there are distinct distances where one sees less change per additional increase in the approach length. This research indicated that approximately 150 feet is that distinct length. Varying the lengths was also found to be more effective when applied to all legs at the same time with the exits.

Findings from this study are intended to provide transportation professionals quantitative means of improving existing roundabout operational performance and also help design future roundabouts with appropriate additional lane lengths that yield better performance. While the design of an additional lane differs from a flared entry, findings from this study can also be applied to flare lengths if they are designed to operate in a similar fashion as additional lane entry.



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