In Ann Arbor Michigan, they've been installing roundabouts in many junctions [1]. My next door neighbor is a landscape architect and is a huge fan of them -- they virtually eliminate fatal accidents. Roundabouts normalize traffic, creating a continuous flow of more evenly spaced cars with overall greater throughput. With islands in the middle, they are much safer for pedestrians.

There are two downsides. They really require much more space than a traditional intersection. They also remove "traffic gaps" between major streets that permit people to make a left hand turn out of a neighborhood.

He's not a huge fan of 2-lane roundabouts. They create an incentive for drivers to cheat by driving down middle of both lanes, staying at 35/40 rather than slowing down to 25 go around the round about. They also cause confusion when someone gets into the wrong lane (left/center lane must go straight through) and this reduces throughput. Perhaps there is a minimum/maximal traffic where they work effectively.

After using roundabouts at nearby intersections, I'd also add that they are much more attractive. Waiting at a light causes stress (and people tend to "text", causing more delays). Given the chance, I prefer to take routes that use roundabouts. I've timed them, and they are faster and more consistent then traditional 4-way-lights, especially at intersections where left/right hand turns normally cause traffic backups.

[1] http://www.annarbor.com/news/roundabout-planned-for-geddes-a...