Roundabouts are ideal when there's a single lane in each direction. Once you've got two or more lanes the roundabout requires crossing lanes on entry & exit if

Roundabouts are ideal when there's a single lane in each direction. Once you've got two or more lanes the roundabout requires crossing lanes on entry & exit if you're not in the outermost lane. That reduces the chance that there's a suitable gap at the entrance, lowering throughput.