BTW a useful site

http://www.viamichelin.com/

as you can see on their map red roads are heavy traffic, yellow moderate, and white low traffic. The thickness on the map corresponds with the road width. Some red roads are indeed shown as wide and named with the D convention, most likely old N roads, but the map is usually correct about the road width information, which is extremely helpful.

When you get on those roads with no pavement markings like the ones i posted above the are generally extremely low traffic, and absolutely wonderful cycling, although sometimes in poor condition surface wise.