I have made gravel roads in a similar fashion. First, I sprinkled some fine sand over where the road was to go, and evened it out with a painters pallette knife (Pallette knives are very cheap at art supply stores and very useful). Then I misted the road with soapy water from a spray bottle. next, i dribbled on a white glue/water/dish soap mixture (the same stuff as for ballast). I added successive layers of sand and glue until I got good coverage, then smoothed the surface with the pallette knife. While the road was still wet, I very gently dragged the head of a screw along the road to simulate tire ruts. If the road is wet enough, the ruts will smooth out a bit, leaving a natural, rounded appearance.
i am happy with the way my road came out, but I may decide to weather it a bit in the future.
kevin