The Region Of Waterloo has a cool site with animated pictures explaining roundabouts (traffic circles):

"Traffic circles have been part of the transportation system in the United States since 1905, when the Columbus Circle designed by William Phelps Eno opened in New York City. Subsequently, many large circles or rotaries were built in Canada and the United States. The prevailing designs enabled high–speed merging and weaving of vehicles. Priority was given to entering vehicles, facilitating high–speed entries. High crash experience and congestion in the circles led to rotaries falling out of favor in North America after the mid–1950’s. Internationally, the experience with traffic circles was equally negative, with many countries experiencing circles that locked up as traffic volumes increased.

...A modern roundabout typically results in less negative environmental impacts than a signalized intersection because it creates shorter delays to motorists and shorter vehicle queues. The longer delays associated with traffic signals result in more vehicles idling for longer periods." (courtesy of Planetizen)