The effects on safety, time consumption and environment of large scale use of roundabouts in an urban area: A case study

An experiment with small roundabouts – as speed reducing measures – was carried out in a Swedish city. The purpose of the study was to test the large scale and long term effects of the roundabouts. The results showed that the roundabouts reduced the speed considerably at the junctions and on links between roundabouts. The lateral displacement the roundabout forces the driver to has a great importance for the speed of approaching cars to a roundabout. The speed-reducing effect is large already at a 2 m deflection. A larger deflection does not result in a larger effect. Conflict studies indicated an overall decrease in accident risk by 44%. Vulnerable road-users' risk was reduced significantly, while there was no reduction for car occupants. There is a relation between the reduction of approach speed and the reduction of injury accident risk. The time consumption at a time operated signal was reduced heavily by the instalment of a roundabout at a signalised intersection. On average, emissions (CO and NOx) at roundabouts replacing non-signalised junctions increased by between 4 and 6%, while a roundabout replacing a signalised intersection led to a reduction by between 20 and 29%. The noise level was reduced at junctions that were provided with roundabout. Car drivers were less positive to the roundabouts than bicyclists. In the long term, the unchanged roundabouts worked almost as well as they did shortly after the rebuilding. The study showed that details in the design are of decisive importance for road-users' safety. Special attention has to be paid to the situation of bicyclists. The transition between the cycle path/lane and the junction has to be designed with care – the bicyclists should be integrated with motorised traffic before they enter the roundabout. There should be only one car lane both on the approach, in the circulating area and on the exit. The size of the roundabout shall be as small as possible.