Hi Frank
If you use the colour output of a PF to drive a Displacement Shader, rather than it's own displacement tab, you can warp that texture by set amounts in each direction around the sample point, then divide the sum of the grey values by the number of the samples taken, to give you an average grey value. Which in essence flattens an area defined.
It's easier to define the road as a straight line along x with a set width in +/- y along the origin, which you can then warp. This allows you to take weighted samples infront and behind to create gentle slopes for the road over your grey terrain, and can create in-cuts and rises. With a little more math you can create drainage ditches as well.
I had a go at this a while back with another forum member, but I've forgotton who I was working with now. Sorry.
I'll see if I can dig out the TGC.