Entry capacity formula of roundabouts in Switzerland.

There are currently about 100 roundabouts in Switzerland. However, 250 roundabout projects are being studied and they will be realised progressively. In order to provide a work sketch which can ensure the quality and the coherence of the roundabout design, a new Swiss guide on roundabout design has recently been elaborated. This paper presents the development of the new entry capacity formula adopted in the guide and is compared with other existing formulae based on field observations in Switzerland. Roundabouts in Switzerland are generally compact (external diameter is about 20-35 m). Field observations at compact roundabouts have confirmed the existence of influence of vehicles leaving by the ious exit on the entry flow. Therefore this influence should be taken into account in the research of entry capacity formula. A microscopic computer simulation model of roundabout entry operations was constructed, in which the phenomena that leaving vehicles disturb entry vehicles can be simulated. This paper describes the development of the simulation model, its validation. The analysis of the influence of leaving vehicles on entry capacity and the determination of the entry capacity formula based on the validated simulation model are also explained. Field observations at 12 entries of 7 roundabouts in Switzerland have been used to evaluate this new formula and compare it with other existing formulae. The comparisons of entry capacity observed in the field and those predicted by the formulae are presented in the paper. The comparison results indicate that the new formula gives better predictions. Finally further research studies are suggested.