At no point in suburban or rural areas did I come across "priorite a droit". In fact in 35 years of visiting France I've probably only seen it 5-6 times and those would have been many years ago
I'm sorry, but I'm really going to have to take issue with this. I would suggest that it's more a case that you've only
noticed priorité à droite 5-6 times rather than actually not come across it. I want this to be very clear - it's one of the reason's why I mentioned it in my post:
priorité à droite is everywhere in France and it is very often not signalled. Over the past 16 years I've lived in Le Havre, Nantes, Paris, Lyon, and now near Amiens, I've also travelled extensively in France and, although it varies from region to region, you cannot escape it. Probably you've been doing what most Brits do, what I did for a long time: sail past them blissfully without even realizing you've not respected the rules, or wondered why someone has pulled out in front of you (but it's true that most French drivers are very respectful to cyclists so this doesn't tend to happen - but it's happened to me a few times when I hadn't realised I'd
grillé une priorité à droite.)
I'd lived here for about two years before I really did start to understand the extent of this rule. I'd known about the marked
priorité à droite but rarely came across them. What took me so long to realise/accept was that this is a fundamental rule on French roads and applies
at every junction unless marked otherwise. And accidents do happen - often. Someone I know was recently involved in a car crash when a vehicle suddenly came in from the right and it was claimed to be his fault because it was
priorité à droite. The fact that he didn't even see the road until it was too late was of no difference.
Some places are more fond of the rule than others. I think it's less common in Britanny - but in the Paris area, for example, it's everywhere (and here you'll still find some roundabouts that are
priorité à droite, which is thankfully rare elsewhere these days
). Also, it's true that this is a common rule that applies in most of Europe not just in France. However, it is particularly frequent here and doesn't seem to be going away (in one of the villages near where I live a road has very recently been changed to become
priorité à droite.)
Just to be even more clear, here are some examples from around my area. It's literally everywhere.
This is the typical scenario:
https://www.google.fr/maps/@49.4478...ata=!3m4!1e1!3m2!1swCvpGoo3jU92o2gX3If-IQ!2e0
This one here is marked. It never used to be. Probably there were loads of accidents at this point:
https://www.google.fr/maps/@49.3194...ata=!3m4!1e1!3m2!1sNe-6_dOz8krQoqPeoQ09Mg!2e0
Usually it's more like this. This junction is only metres from the other, going the other way (obviously you'd be on the other side of the road). Notice there's no line on the road branching off from the road you're on here - that means priorité à droite. Still, at least you can see this one ... often that's not the case until you've gone past it:
https://www.google.fr/maps/@49.3188...ata=!3m4!1e1!3m2!1ssejMmxCgFgUYJ_YGs2XQug!2e0