The purpose of the study was to test the large scale and long term effects of roundabouts in a Swedish city. The results showed that the roundabouts reduced the speed considerably at the junctions and on links between roundabouts. Conflict studies indicated an overall decrease in accident risk by 44%. Vulnerable road-users’ risk was reduced significantly. 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 a signalised intersection led to a reduction by between 20 and 29%. The noise level was reduced at junctions that were provided with roundabout. 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.