The US is only just starting to get round abouts (AKA Traffic Circles). I think traffic circles can be a great thing, but in my experience, most aren't done as efficiently as they could be.
I drive through 4 traffic circles on my drive home each evening, and here are some observations I've noticed.
Size Matters
The larger it is, the more opportunity a driver has to get to an internal lane and not destroy on-ramping from other directions. Also the timidness of drivers to enter a circle kills the flow and causes a backup that takes time to clear.
Of the 4 traffic circles I drive through, two of them are only a single lane wide and only about 100 feet wide, the other two are much larger and have 2 and 3 lanes.
The narrow ones [1] back up much quicker during rush hour(s) especially when you have non-commuters using them (you can tell who they are).
A Minimum of 2 Lanes, but 3 is LOADS Better
The one that is only two lanes [2] backs up quite a bit during rush hour, when there is a West Point football game, or because someone is too scared to enter. The backup clears relatively quickly, but I feel it wouldn't back up at all if there was protected on-ramp.
Protected Entrances and Exits Increase Flow
The one that is 3 lanes wide [3] usually (when people don't cross solid white lines illegally) has a protected lane for only going to the first exit (directly on then off), then an inside lane that circles the entire roundabout and a middle lane that allows you to exit at any of the exits whenever you want. It works very well. I have never seen a backup on it.
