To be more specific, a D road is maintained by the Department, ie a sort of local council, and can - and often does - change number as it crosses a local council boundary (this can be very confusing).
N roads are maintained by the National government, while A roads are autoroutes and you have to pay a toll for (nearly) all of them. So a D road could be really busy - for instance one by the coast in the summer such as the road south from Boulogne which is very busy but also has a good cycle route by it all the way - while an N road may be quiet.
But by and large, there are so many roads in France they they tend to get less use, and they have much less frost damage than UK roads so are often in better condition. Having said that, they do have their fair share of farm tracks masquerading as roads.
Oh - and I have a mirror on the bike, to tell me which side of the road to ride on. Strange, but true.
this is starting to look good. I'll pack my budgie-smugglers.