Capacity of Single-Lane Roundabouts with Slow Traffic

Many roundabouts have been built in the Netherlands over the last 15 years. A much-debated issue has been whether to give slow traffic, especially cyclists, priority. Recently, a general recommendation was made: priority for slow traffic inside built-up areas and the reverse for rural roundabouts. Because the implementation of this recommendation could result in a large number of roundabouts with priority for slow traffic, there was a need to develop a method that determined the capacity and delay, taking the influence of slow traffic fully into account. In this study such a model was developed. It is an analytical model that combines parts of existing analytical submodels, with extensions. Motorized traffic entering a roundabout is confronted with three conflicting streams: the well-known circulating stream of vehicles on the roundabout, the parallel slow traffic blocking the entry, and the circulating stream of slow traffic at the next downstream exit that blocks the outgoing motorized vehicles and as a consequence can also block the entry. The model was calibrated successfully with observations from two one-lane saturated roundabouts. With the model not only the capacity per entry can be calculated but also the capacity of the total roundabout, given a relative origin-destination matrix of fast and slow traffic.