Here's an unreliable source: "The overriding rule of the road in France is Priorité a droite which basically means that you must give way to traffic coming from your right. It is an archaic law that dates back to the times of the horse and cart and for some unknown reason has never been repealed. It is true to say however that the French have spent thousands of millions of Euros to indicate that it does not apply in most circumstances. We have tried to explain some of the mysteries surrounding this law, but the old driving proverb is very apt here. "if in doubt put both feet out" i.e. Slow down or stop."
From http://www.vendee-guide.co.uk/priority-a-droite.htm
(That's not good advice, obviously, as if it was the only principle you knew it would lead to hopelessly tentative driving.)
A slightly more comprehensive source: http://www.france-pub.com/forum/2012/04/14/la-priorite-a-dro...