It's wise to buy a detailed map of the areas you plan to tour; these will also show you the smaller D roads, which are even more pleasant.
It is unwise to invest in a map when touring a new place. 
mnli hypothesis: It is unwise to invest in a map when touring a new place.  premise: It's wise to buy a detailed map of the areas you plan to tour; these will also show you the smaller D roads, which are even more pleasant.