Yep, in my city we have a roundabout with 4 cardinal directions, roughly aligned with compass directions.

The West connection is to/from highway, so it has most traffic. People arriving from North/East/South want to leave on the West. And this causes massive jams, as the constant supply of traffic going from South entry to the West exit (i.e. doing left turn, and passing all ramps) essentially blocks all the other traffic.

Roundabout are great, they increase safety of dangerous intersections. But sometimes a controlled intersection is just a better idea.

Have you measured it?

No, I got that from a materials road engineers made available publicly a while ago when I was interested in the topic. It had whole bunch of various trade-offs you can make with intersections in it.

Can we see it?

the UK has traffic lights on certain heavily used roundabouts - sometime the lights will be temporary and only turn on once the throughput hits a certian level

I believe that I've read that elsewhere. They allow for better and higher flow than an intersection, but have a weird failure mode at very high rates where the circle gets loaded and all directions thus stop.

It's rather that if there's a continuous inflow of traffic that usually doesn't take one of the exits, then all traffic coming coming from that exit doesn't get a chance to join the roundabout.

What usually happens is the roundabout gets traffic lights.