@PAD-0 yes, it is totally illegal in France.
However, just being back from two weeks in France, it appears as if the speed limits have been spread over the roads with a random number generator. Only logic remaining is that in cities or villages; 50km/h is the standard, if not indicated differently as being 30km/h, and I also saw 70 km/h in the middle of a city, and the best one: a white painted 50 km/h on the road with a sign 70km/h right next to it.
And what about a road in the middle of nowhere with a roadsign for 90 km/h in one direction and 70 km/h in the other direction.
This seems specially designed to create confusion and generate income for the state in combination with a massive amount of speed cameras (even in the middle of forests or mountain roads).
Having some protective equipment bcomes self-defence.