Details from Roundabout USA

Roundabouts require entering drivers to give way to all traffic within the roundabout, regardless of lane position, while traffic circles typically allow traffic to enter alongside traffic circulating in an inner lane without consequence.

Generally, exiting directly from the inner lane of a multi-lane roundabout is permitted, and such exiting traffic has the right-of-way over entering traffic. By contrast, exiting from the inner lane of a traffic circle is usually not permitted without first executing a lane change to the outside of the circle. Massachusetts, where older traffic circles, or rotaries, are commonplace, has a dwindling number of exceptions.

Deflection on entry is used to maintain low speed operation in roundabouts. Drivers must manoeuvre (are deflected) around the splitter islands and the central island, at speeds of 15–25 miles per hour (24–40 km/h). Many older traffic circle junctions allow entry at higher speeds due to the lack of deflection, or require a stop and a 90-degree turn to enter, creating a large difference in speed between entering and circulating traffic which can make it difficult for entering drivers to find suitable gaps in heavy traffic.

Pedestrians are usually prohibited from the central island of roundabouts, and the crosswalk for pedestrians and some cyclists is withdrawn from the junction by at least the length of one vehicle.

All vehicles circulate around the central island of a roundabout in the same direction, which is determined by whether traffic drives to the right or the left. In left-hand traffic countries the circulation is clockwise; in those that drive to the right, it is anticlockwise.