Problems of ensuring traffic flow on a Basic turbo roundabout with non-standard curvilinear entries (a case study)

Abstract: With ever-increasing motorisation and the desire for greater traffic safety, conventional roundabouts and turbo roundabouts are being designed. Turbo roundabouts provide greater traffic safety than conventional roundabouts due to the segregation of traffic at the entries and on the roadway. In the design of a turbo roundabout, the usual roundabout principles recommended in Dutch guidelines can be applied. However, these apply when the roundabout entrances are directed perpendicularly to each other, which is not always possible. This paper undertakes an analysis of the traffic flow on a Basic turbo roundabout with curvilinear entries directed at an angle other than a right angle to each other. A junction in a built-up area located on the outskirts of Szczecin was selected for analysis as a case study. In the first phase of the turbo roundabout design, the recommended roundabout parameters given in the Dutch guidelines were adopted for a roundabout with a 0.7 m wide separation lane. After preliminary swept path analysis of the accepted design vehicles, it became necessary to design the entries individually due to their curvilinear nature and angle. The analysis of the swept paths provision at the Basic turbo roundabout with a non-standard orientation of the curvilinear entries shows that wider lanes and larger corner radius than recommended in the Dutch guidelines should be used. Higher angle splitter island should also be considered. Keywords: Turbo-roundabout; Raised lane dividers; Swept path analysis; Curvilinear entries; Fastest-path speeds