You over estimate the intelligence of the average American. I've lived in a few cities with a number of roundabouts and while I love them, the number of stupid people that panic and..

-stop in the roundabout

-stop before the roundabout and let their brain buffer for 30 seconds.

-somehow go the wrong way in the roundabout

-fail to yield to traffic in the roundabout

Is way too damn high. It makes traversing one a high stress situation since you have no idea if grandpa grunt and run in to you is about to perform a confusion based terror attack on the traffic control device.