Congestion on roundabouts is often caused by a steady, but often relatively small circulating flow making entry of vehicles difficult. Traffic signals can help in two distinct types of situation: (a) where a flow imbalance prevents easy entry by a single major movement; and (b) where the roundabout is operating near capacity on several approaches. A solution to (a) is to signalise the major movement entry on a part-time basis. Optimising the signal timings may be achieved by modelling the junction as an isloated `T' junction. A solution to (b) is to signalise most, or all the entries on a full-time basis. Combined with appropriate geometric changes, capacity can be improved. The signal timings are critical though and fixed time plans are usually appropriate. In Great Britain, the modelling program TRANSYT is used to optimise the timings, but there are special requirements to obtain the desired results, and these are presented. (A)
Abstract