As a rough guide:
N = nationale, equivalent to A roads, would avoid but some are OK
D = Departementale, Equivalent to B roads, go for it!
VC = vicinale equivalent to unclassified but generally well maintained
Voie verte: Equivalent to NCN, no motorised traffic.
Cyclists are not allowed on most dual carriageway Nationales, but by law they have to provide a parallelish road. Very often it is the old nationale so very good (wide with low traffic).
This sign means motorised traffic only: