The city of Forest Lake, Minnesota is going to deploy a slew of roundabouts to control traffic, replacing traffic lights. This article describes the plan: St. Paul Pioneer Press | 09/09/2006 | Roundabout plan may get a trial spin
In most studies of the issue, roundabouts improve on safety and reduce delay compared with traffic signals. They have the advantage of minimizing conflict points at intersections, and help keep drivers moving, while slowing them down and making them more alert.
This wikipedia article describes roundabouts in more depth.
Unfortunately, traffic circles have given roundabouts a bad name, and becaues they are not so complicated, they are often not even taught in Traffic Engineering classes, so they are not given due placement in the traffic engineers toolbox.