A Mixed Method Approach for Modelling Entry Capacity at Rotary Intersections

Rotary intersections, known as old traffic circles, require vehicles entering from branches to yield to circulating traffic. Upon entering, vehicles travel around a central island and exit toward their desired branch, generating merging and diverging conflicts at entry and exit points. Rotary capacity models are focused on the weaving manoeuvres within the circular roadway sections, associating capacity with the maximum traffic flow rate of each weaving segment. This paper introduces a novel approach combining modern roundabouts capacity models with the old rotary ones. In particular, the present study proposes a mixed approach based on an iterative process that combines the English TRRL model, which is suited for the old rotaries and based on short weaving sections capacity, with the features of the HCM-7th entry capacity model of the modern roundabouts, which is based on the circulating-traffic priority rule. Such an approach is rooted in the total capacity criteria and traffic conditions where all roundabout entrances reach congestion simultaneously. Compared to the past, this new approach makes entry performance estimation, such as average delay and queue length, bridging the gap between outdated and current methodologies in the field of rotary intersection design and assessment.