How rough a road are you trying to make? If it is fairly flat, how about using some fine sandpaper, over-painted to get the color you want. The other thing is how many "dirt" roads are actually dirt? Most I've seen are really gravel, except where you get into a farm yard where it may be dirt. Even then, the color may vary. In the mid-West, where the topsoil is fairly dark black, a dirt road may, if dried out, be some shade of dark gray. Out West, or in the Southwest, it may be more sandy color. Around an industrial area or railroad yard, grease, grime and oil, combined with cinders may have it a flat black. Just a thought...