A roundabout, a rotary and a traffic circle are types of circular road in which traffic is permitted to flow in one direction around a central island.
In the United States, engineers use the term modern roundabout to refer to junctions installed after 1960 that incorporate design rules to increase safety. Compared to stop signs, traffic signals, and earlier forms of roundabouts, modern roundabouts reduce the likelihood and severity of collisions greatly by reducing traffic speeds through horizontal deflection and minimising T-bone and head-on collisions. Variations on the basic concept include integration with tram or train lines, two-way flow, higher speeds and many others.