The capacity of roundabouts

A roundabout is one of the important types of unsignalized intersections. It is used in many places. The calculation and appraisal of the capacity of roundabouts need further research. Usually the traffic rule in a roundabout is that the vehicles out of the roundabout have the priority. From the hypothesis that the headtime of the roundabout obeys the Erlang distribution and by using the clearance-interlude-theory, the paper deduces the calculation formula of the saturation flow of the into- roundabout lanes. By using queuing theory, the paper provides a method to appraise the conditions of the into-roundabout lanes, and deduces that the appropriate ratio of the design capacity with the theory capacity is between 0.5 and 0.8.