Breakdowns Broken down trains affect one line or all left turns.
Split Before Merge
Can handle lots of trains moving N-E or S-W
Tracks split and only trains going to the left need to take the round-a-bout
To allow left-turning trains traveling at full speed through the roundabout, it's possible to enlarge the circle, but this takes a lot of space to build.
If you make the inner circle run the opposite way (obviously modifying exit diagonals and signal direction) throughput is increased at the expense of some speed, since trains going up there just cut across instead of having to go round the whole roundabout as tey would with the above.