Very simple
by using a simple shape shader. Make its width 5m, set the edge width to 0.5m or so. You may want to warp this by using a warp and redirect shader (plug a PF with displacement into the X or Z of the redirect, and change the position key in the SSS to position in terrain, or it won't warp). Simply said. This is your road mask. Use it to blend your series of small fake stones as gravel. Use it as an inverted blender for your populations of grass and trees et voila.
