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.

Have you been on a congested highway or interstate?