Roundabouts have a 'window of efficiency' where light-to-moderate irregular traffic works wonderfully well. But when an area grows and traffic increases, it can move out of this window. When you have regular moderate to-heavy traffic, it can be impossible to get onto a roundabout when facing say commuter traffic in the other direction.
This is probably why rural American cities are installing roundabouts at the same time urban British cities are demolishing them.
This is probably why rural American cities are installing roundabouts at the same time urban British cities are demolishing them.