It boils down to looks vs function IMHO.
Personally I tend to go for looks when I draw a map. Therefore for "tertiary" roads I use secondary roads and less frequently bare earth, which tends to look good only on a few terrains (forest cover etc.).
If instead you put an emphasis on game mechanics the others solutions may be more valid even if personally I find them an eyesore.
Okay, the poll is over and I can see the result is quite clear - thanks you for your input, will modify my archive maps accordingly and will keep it in mind for future maps.
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This post, as well as being an ambassador of death for the enemies of humanity, has a main message of peace and friendship.
It's late for the poll, but I throw my 2 cents in anyway.
why not leaving tertiary roads plain flat terrain while adding high grass/crops/leaves/rough/sand/whatever to non-roads hexes representing the country? This would make it for the look and for the MP cost where a difference is needed.