Here's a pretty good article explaining what makes a roundabout different from a rotary/traffic circle:
http://www.theunionleader.com/articl...4-39e03b7effc6
They really are quite a bit safer than a signal. They may not seem it because people seem to get confused sometimes, but you've effectively eliminated all risk of a high speed angle collision, which tend to cause many of the fatalities at intersections. Also, the lower speed (resulting from the smaller diameter) also increases capacity. It seems counter-intuitive, but the slower you drive, the closer together you drive, so more cars can actually pass through the circle at a time.