Best way to do what Tonden is asking would be to make an actual series of road tiles where the ruts/tracks were slightly depressed into the geometry of the ground and textured with a dirt, mud, or gravel texture. The walkmesh between the tiles can then be set to grass material, leaving the ruts/tracks as dirt material and the areas adjacent to the roads as grass material. In this manner you would get an appearance identical to the picture Tonden posted.
The ruts/tracks could be a simple V track cut into the ground mesh and detached as a separate object. This would create the desired effect, still be low poly, and not require any special modification to the Z height of the walkmesh to accommodate the V-shaped cutout tracks. If you want something more realistic for the ruts, use a low-poly U-shaped scallop - probably a 6 face semicircle would suffice.
In essence, the geometry is an inverse of what I did for the Q ploughed fields. The furrows on those tiles are actually low-poly triangular peaks whose tips are slightly higher than the surrounding ground.
[EDIT]
There is also an easier way I just thought of if you don't mind sacrificing the 3-D geometry of the above method, but it requires a little work lining up the texture with the walkmesh...
If you just want to change the material in the walkmesh, all of the road tiles have a wide swath of dirt material that corresponds to the location of the road texture on the tile. All you would need to do is reconstruct that section of the walkmesh, adding a strip down the middle of the track and setting the material of the newly created faces to "grass". The easiest way to do this would be to...
1. Select the faces of the dirt material on the walkmesh and delete them - you'll be left with two outer faces of "grass" with a void down the middle
2. Clone the outer vertices on each edge of the deleted track and slide them toward the middle, creating a narrow path down the middle of the track
3. Create new faces, filling in the middle of the empty void created when you deleted the original dirt material faces from the object - you'll now have two strips of grass faces on the left and right, a thin void (where the track will go) on the left and right, and then a thinner strip of grass faces in the middle of the two thinner voids
4. Select the newly created faces and set the material to "grass"
5. Create new faces, one on each side of the newly created strip of grass faces.
6. Select the faces of each thinner strip and set the material to dirt (or stone if you prefer)
The walkmesh will look something like this when you are done...
[/EDIT]
