I checked out the
of the simulation. In heavy traffic, the roundabout will be full and approaching traffic will back up, waiting for an opening. In this case, with two lanes in the roundabout, it doesn't matter which lane has a vehicle in it, the approaching vehicles must assume any vehicle in the roundabout will veer to the right and therefore must wait until it passes.
Again, in light traffic, roundabouts work great., but in heavy traffic you get gridlock.