Roundabouts have a &#x27;window of efficiency&#x27; 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.<p>This is probably why rural American cities are installing roundabouts at the same time urban British cities are demolishing them.