[Toro]

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I kind of figured that the reason for their popularity was cost relative to traffic lights. As for "keeping traffic moving" I suspect that this is highly dependent on traffic volume. I haven't really observed this, but I suspect that at a high enough volume, they will rapidly switch from "keeping traffic moving" to "backing traffic up in all directions". Of course, traffic lights will back up traffic pretty well with heavy volume, too. As you may have guessed from my post, I don't like them much.

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There is also a new purpose for roundabouts. It's called "traffic calming". Roundabouts are installed at intersections as a means to slow down traffic. I would never approve one here, but that's what they do in some places.