Roundabouts are not applicable to dense urban areas or highways, which is where the traffic is.
Even if this is so, a computer-driven cars can certainly use normal junctions much more effectively than humans. Robots could do this: http://www.youtube.com/watch?v=Vnba7Y86jC4 [lucky driver goes full speed through dense traffic] on routine basis.
Seriously doubt it, and I don't want to live in that world...
In any event, it's as likely as seeing people riding segways all over the place.
Have you been on a congested highway or interstate?