My rural county has gone wild for roundabouts. It appears we got a new traffic engineer about five years ago and they've set about to make as many roundabouts as possible. We even have a double roundabout now. I'm surprised by how little controversy it created and how quickly everyone adapted. I can only think of one that hasn't worked out well and that's because traffic from a very close Chick-fil-a ends up back up into the roundabout and clogging it up.
Due to the rural/exurban character of the area, space for roundabouts isn't much of an issue. I suspect lots of other similar counties could do the same with similar results but it takes someone in the planning office to take the chance on doing it. Also it takes money so from a fiscal point of view, it's best to redo intersections when they're already scheduled for being repaved or they're having capacity issues that need to be addressed.
My rural area has rules against roundabouts. Depending on dimensions, they can be difficult for large farm machinery to negotiate. And in in the winter they are more difficult to plow.
For reasons that are unclear, a lot of roundabouts in the U.S. are so small that its nearly impossible for two vehicles to be in the roundabout at the same time.
Which makes them basically useless, and difficult navigate for e.g. vehicles with trailers--you end up running dragging the trailer over the center or something.
There are a lot of these in Berkeley. On top of being small, they have stop signs at the entrances, too! I tend to believe they're mostly an excuse to put a tree in the middle of the road, because a lot of them do have trees in the center.
That's actually measurable. You take the data from before the change and the estimations (based on similar changes elsewhere) and plot how long it takes for the traffic to reach expectations (in terms of things like throughput and decreased collisions). With that, you can find out how long the average drivers took to adjust.
Due to the rural/exurban character of the area, space for roundabouts isn't much of an issue. I suspect lots of other similar counties could do the same with similar results but it takes someone in the planning office to take the chance on doing it. Also it takes money so from a fiscal point of view, it's best to redo intersections when they're already scheduled for being repaved or they're having capacity issues that need to be addressed.