When I first came here they usually had three lanes, with the idea that you change lanes as you move through the roundabout - this meant almost daily you would see a minor RTI on a roundabout, which would cause large queues of traffic. Some roundabouts also had three lanes where you enter and exit, but they were usually signalled.
A few years ago they reduced the roundabouts to two lanes, and all exits to one lane. The idea being that for the first and second exit you need to use the outer lane, and for the third or fourth exit you use the inner lane. That inner lane becomes the outer lane after the second exit, so you just stick to the same lane once on the roundabout. The only time you need to change lanes is if you miss your exit and need to go around again. There are clear signs before you reach the roundabout saying which lane you need for which exit.
Since this change was made, I think I've seen maybe one RTI on a roundabout - when somebody was being stupid and tried to overtake a bus. It's such a simple change, but it's crazy how effective it has been. They've also started adding speed bumps to some roundabouts to slow traffic, which actually increases capacity as it's easier to join with slow moving traffic.