Well, I tried painting a road (green) on my enviro's RGB discriminator map, then with a finer brush painted it back to black down the center of the green road path.

And here the RGB is overlaid as ambient on the terrain to quickly show where the different material types will go...

And then ran a test render;  the result is a road with two wagon wheel tracks.

So now I've got a two-horsepower elvish carriage under construction.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5