A roundabout has conventions for who has priority on enter/exit which means one of the lanes waits for a space in the other.
Motorway junctions and this example have the joining lane separate from the existing one so either can block the other.
Note that high traffic roundabouts usually end up with traffic lights to force a degree of fairness, where relative flow rate can be adjusted by different timing on the lights.
It's the difference between enforcing mutual exclusion by control flow vs a futex.
