Roundabouts are awesome --- self-organising, self-managing, power-free traffic junctions! But they do have a failure mode, which is that they tend to starve low-traffic loads if the traffic is too asymmetric.

For example: consider a T-junction with a roundabout at the junction. (Very common in the UK.) Most of the traffic is travelling across the T in a straight line. But I'm entering the junction via the cross road.

The rules are that traffic from the driver side has priority over me, but that I have priority over traffic on the passenger side. So I stop at the line and wait for a gap.

With balanced traffic, I'll be guaranteed a gap because the oncoming traffic will have to stop to wait for someone else, which means I can move onto the roundabout ahead of the vehicle they're waiting for. But with unbalanced traffic, there won't be a gap, and I'll be stuck.

Typically what the traffic planners do at this point is add traffic lights, which is basically giving up, and defeats the whole point of having a roundabout in the first place.

...I used to live near this monstrosity, which had lights at every stop line. Blech. https://www.google.co.uk/maps/@51.436168,-0.8971696,200m/dat...