"Traffic already on a road" is a concept that does not apply in this scenario, as all traffic is already on a road in all cases. There is no a priori way to dis

"Traffic already on a road" is a concept that does not apply in this scenario, as all traffic is already on a road in all cases. There is no a priori way to distinguish between the circular road and the roads leading towards it. In countries where you drive on the right, I believe the typical default rule is that traffic coming from the right has priority. In roundabouts, this would mean that traffic on the roundabout must wait for those joining it. However, the rule is reversed for roundabouts. (This definitely applies to Germany, and I suspect that it applies in most other countries as well.)