A modern roundabout is a safe and efficient form of traffic control which is relatively new to the United States, although very common throughout other portions of the world for several decades as an alternative to stop-controlled and signalized intersections. The main characteristic of a modern roundabout is the “yield-at-entry” rule, meaning that traffic entering a roundabout must yield to the traffic already within the roundabout. Roundabouts are designed for lower speeds and can have one or multiple travel lanes depending on traffic volumes. Studies have shown that roundabouts can greatly improve the capacity and safety of intersections when compared to stop signs and signals. This is primarily because of lower entry speeds, simplified decision making and fewer conflict points. Four way intersections have 32 possible vehicle to vehicle conflict points, but roundabouts only have 8, as illustrated in the below drawing.
Modern roundabouts should not be confused with the older style traffic circles. The design of traffic circles was based on larger diameters and high-speed entry and exit speeds.