Analysis of roundabout capacity under high demand flows

Estimation of the entry capacity of roundabouts greatly varies between one method and another in their input requirements, model complexity and estimate accuracy. Several available models for the capacity estimation at approach entries of the roundabouts are tested here and compared for high traffic demand condition. The estimated entry capacities along with the actual entry demand are then compared for various gathered circulating flows. Two models for capacity estimate are developed for triple circulating lanes by triple and dual entry lanes. Thirteen roundabouts in Bahrain are used for the models’ development and for the comparison purposes between the various tested methods. The geometric data were gathered from the actual drawings, scaled aerial photographs and from the field. The necessary traffic data were gathered during peak periods. Substantial differences in estimating capacities were observed between the various available international methods. This makes the judgment of accepting or rejecting the estimated capacity difficult. The developed model fall well in between the tested international models and matched the actual data reasonably well. The methods with complicated input parameters and extensive equations, as aaSIDRA, UK RODEL, French and Indian methods, did not prove to be better than much simpler ones like Swiss or FHWA methods. 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.