Piers, what kind of road island has traffic lights? Is this some sneaky way of getting more of those damn light-controlled intersections into use to plague us?
Seriously though, a well designed roundabout/road-island is the most efficient way I've see of moving light to medium traffic through an intersection. They do have failings, in heavy traffic fortune only favours the brave, and as Liz points out, they are impossible to cross. However, in heavy traffic nothing much moves at traffic lights because the whole lot is blocked up with cars stopped by the NEXT traffic lights. And bloody pedestrians crossing. (Hey, they were born on that side of town.....)

I don't like the little roundabouts they have here in Auckland. You can go through them at 50kph, so people DO enter them at 50kph. Or 60kph. Looking right. With the next entry 5 metres ahead.
THATS the way to design a roundabout to give roundabouts a bad name!