Capacity and design of traffic circles in Germany

Abstract

Capacity and traffic safety of traffic circles in Germany have been investigated in comprehensive research projects carried out at the Ruhr University Bochum. On the basis of observations of real traffic conditions, formulas for predicting traffic circle capacity under conditions in Germany have been developed. These formulas have been specified in terms of the number of lanes in the circle and in the entry. The capacities of traffic circles measured in Germany appear to be considerably less than the values predicted by English formulas. For German traffic circles, capacity has been measured at between 0.7 and 0.8 of English values. However, there is good agreement between calculations for French and German capacity. The influence of geometric parameters was also investigated. In addition to the number of lanes in the circle and in the entry, geometric parameters turned out to be significant for estimating the capacity of traffic circles. These parameters are inscribed circle diameter, number of traffic circle arms, and distance between exit and entry of the observed arm. Traffic circles in Germany also provide a considerable increase in traffic safety when compared with normal signalized or unsignalized junctions. Before and after investigations at small, single-lane traffic circles showed that traffic circles have a lower severity of accidents, whereas the number of accidents stayed almost the same. Experience with existing traffic circles resulted in some significant recommendations for the design of traffic circles. Using the suggested design criteria, good experience has been shown in several cases and has led to a rapid increase in the number of new single-lane traffic circles. Public opinion also favors traffic circles.