Abstract
In roundabouts carrying high volumes of traffic, congestion frequently occurs to such an extend that the roundabout becomes completely blocked. Traffic signals on each approach have been used experimentally to moderate the inflow of traffic, but to ensure that locking would hardly ever occur it would be necessary to reduce the inflow severely. The active describes the successful application of a congestion detector to a signal-controlled roundabout. The system operated by modifying the all-red period so that the inflow of traffic was reduced when congestion was detected in the roundabout. A description of the detector, its operation and installation is given, together with the results of some "before" and "after" studies.