TRAFFIC SIGNAL CONTROL OF ROUNDABOUTS

Full signal control of traffic was introduced in 1978 at a 35 M diameter, three approach roundabout near the centre of Nottingham. This paper describes the development of the control method giving formulae used to determine cycle times, green times and junction capacities. Details are given of the sequence of traffic movements which allows at least one traffic stream to be entering the roundabout at all times within the cycle. Green periods for each entry slightly overlap allowing simultaneous entry for two streams over short times. Results of before-and-after surveys indicate that the entry capacity was increased by 10 to 40 percent according to the time of day, and that the flow could be significantly increased by junction control if it were accompanied by physical changes in layout. Other benefits include reduced delays and better overall control of the junction using urban traffic control systems.