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An exponential model is developed for capacity estimate of roundabouts with triple circulating lanes based on given circulating flows. The necessary data for the development of the model and for the comparison purposes between the model and other international models are gathered from 13 roundabouts in Bahrain. The geometric data were gathered from the actual drawings, scaled aerial photographs and from the field. The necessary traffic data were gathered during morning and evening peak periods. The developed model falls well in between the tested international models and matches the actual data reasonably well. Substantial differences in estimating capacities were observed between the various available international methods. Such variations make the judgment of accepting or rejecting the estimated capacities difficult. They will also make the gating strategies and traffic assignments unreliable. The methods with complicated input parameters, extensive equations and tedious calculations, such as aaSIDRA, UK RODEL, French and Indian methods, did not prove to be better than much simpler ones, such as HCM or FHWA methods, in estimating roundabout capacities when compared with actual data. The influence of most of the geometric parameters of roundabouts on capacity during rush hours is limited. The findings are quite essential for traffic planners in making judicious decisions regarding roundabouts’ performance. There is a real need for a more consistent model for the capacity estimation of roundabouts.
circulating or conflicting flow, roundabout capacity, triple circulating lanes, entry capacity
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