Priority to the right: the "Priorite a droite"
This strange rule only applies when driving in France, so it causes much confusion for forign drivers. This rule basically states that when driving along a road, anyone joining from your right hand side has priority over the main road on which you are driving, regardless of the size of the adjoining road. The joining car does not have to stop - you do! This law is thankfully not widely used any more, but in certain roundabouts (like some main roundabouts in Paris for example) you will still see cars stop as the traffic from the right joins. On the expressways this rule doesn't apply anymore.
http://www.driving-in.com/france/

In Oregon (as I think all of the US) merging traffic must match speed or yield. The vehicle in the roundabout has right of way.