I agree: on the map scale only landscape features would affect the straight line of a road, on the zoomed in scale everything from a boulder to muddy patch can divert a track. On the map scale a scematic representation may be of more use than an aestetically pleasing dirt river.
mountains, cliffs, dense growth, agressive localized wildlife, etc deters roads, while settlements, freshwater features, etc attract them.
To see track formation, you can look at places in suburban areas where footpaths or bike trails take square turns and have only grass or low growth on the inside of the corner (no curbs or fences to deter shortcuts). In due time enough people will cut the corner that a dirt track will form, often several progressively outward.
Or when a shortcut is possible through a small thicket, say from a schoolyard to an interesting pace such as a convenience store, several competing paths will evolve inside the thicket, skirting large trees and avoiding thorny bushes, diverging and merging.
It would be great if traffic would create the paths, as in Dwarf Fortress for example, but not truly neccesary. It does require each individual moving unit to calculate its optimal path, which is in DF a major drain on the system resources. But DF has collisions and blocking. Caravans/units, I feel, could be able to move fairly unimpeded by each other (maybe a small speed penalty for crowding) to avoid having to recalculate their optimal path each time they meet another unit on their path.
