Roundabouts (or rotaries, or traffic circles, as they're known in parts of the U.S.)
This is part of the problem with the perception of roundabouts in the US: people don't know what is and what is not a roundabout. The modern roundabout came about in the UK in the late 70's and early 80's, and is a small (typically <200' diameter, depending on the number of lanes) circular intersection where the entering traffic yields the right of way to circulating vehicles. In multi-lane roundabouts, lane changes within the roundabout are not permitted.
Rotaries and traffic circles have been present in the US for the past 100 years (and are much more common on the east cost than elsewhere). These can be up to 1000' in diameter, and have varying traffic control (from circulating vehicles yielding to entering vehicles, to traffic signals at the approaches or crosswalks). Speeds at rotaries and traffic circles can also be quite high, and lane changes and merging are often permitted.
The main difference is that the modern roundabout is designed to keep traffic speeds low (ideally around 15mph, but usually less than 25 mph), and are often empirically designed (this varies from state to state, many use empirical models, but some use gap acceptance theory) based upon driver behavior.
But I loathe rotaries when there's a lot of traffic. You can sit there for a lot longer than you would at a red light.
Roundabouts are not indented for extremely high traffic volumes, or in close proximity to signalized intersections, as the platooning vehicles from the adjacent signals will disrupt the flow of traffic through the roundabout. If you are stopped and waiting at modern roundabout, unless the traffic condition is extremely uncommon for the location, the roundabout was either designed improperly, or was ill-considered for the location. If you are not in fact at a modern roundabout, but at an old-style rotary or traffic circle, then the FSM only knows what is happening.
And, BTW, I am a highway engineer, so take this as you may.