How big are the roundabouts? I've read one of the problems that can be had is smaller ones, like in this picture, allow drivers to go through at high speed unimpeded. The "ideal" design seems to be to wiggle the road the opposite direction of the turning motion then force going around a decent sized center. In this way going around a roundabout quickly isn't impossible... but going 35 mph would make most feel quite uneasy. The downside is it greatly amplifies one of the biggest roundabout downsides: the amount of space needed.