The impact of exiting flows on roundabout lane entry capacity

The traffic performance of a roundabout depends on the accuracy of the capacity models used for its design and analysis. However, the inconsistencies among the inputs for various existing models suggest that our understanding of the variables which impact on roundabout capacity remains limited. This paper thus presents the results of an investigation into the impact on entry capacity of the flows exiting on the same arm as the subject entry, as recent empirical research showed that this variable could explain more variation in entry capacity than other more commonly-used variables. Microscopic simulation models were developed for three entries of different-sized roundabouts, calibrated using ground-truth capacity data and follow-on headways. Experimental investigations revealed piecewise linear relationships between separation and capacity, which affected the direction and magnitude of the effect of exiting flows on capacity. At low separation distances, inhibitory effects (where the inability of entering drivers to distinguish circulating vehicles from exiting vehicles leads them to inadvertently reject acceptable gaps either wholly or partially) dominated and resulted in negative impacts by exiting flow. However, at separation distances larger than a critical distance threshold, exiting flows had near-linear positive relationship with entry capacity, arising possibly from their correlation with the proportion of circulating vehicles originating from the first entry immediately upstream. A new model was hypothesised based on these observations, and regressed on a large set of recent field data. The resulting new nonlinear regression model was found to perform well and provided a better explanation for the impacts of separation and exiting flow compared to those developed for the same dataset in a preceding study. By enabling a better understanding of the impacts of separation and exiting flow on capacity, this study hopes to contribute towards the development of improved roundabout capacity models.