The Project for Public Spaces has a neat post on how lots of towns are replacing intersections with roundabouts. Not only can roundabouts cut down on congestion by up to 20 percent, but they can reduce the number of crashes at intersections by as much 75 percent, as compared with traditional stop signs or traffic lights. Several reasons for that:
1) Low travel speeds—because drivers must yield to traffic before entering a roundabout, they naturally slow down,
2) no red lights to run—roundabouts are designed to keep traffic flowing without requiring vehicles to stop, so the incentive for drivers to speed up to make it through a yellow or red light is removed, and
3) less potential for serious crashes—since vehicles all travel around the center island in the same direction, head-on and left-hand turn (T-bone) collisions are eliminated.
Roundabouts, by the way, are slightly different from the traffic circles we have all over Washington D.C. With traffic circles, vehicles inside the loop don't always have the right of way—especially if there are stoplights, as you see in Dupont Circle and elsewhere. Large traffic circles have an inherently flawed design and can often make congestion even worse, which is why planners swore off building them after the 1950s. But modern-day roundabouts, in which incoming traffic simply yields to cars already in the circle, work quite smoothly, which explains why they're creeping back in fashion. Obviously you can't just plop these things down everywhere, but in certain locations they make a lot of sense—and, as a bonus, they can be more attractive than typical four-way intersections.