I've already my roads most of the time hovering above the terrain but still sometimes the terrain temporarily "shines" through depending on distance and view angle. I guess it's due the terrain's LOD, so some polys might become higher than intended on some LOD levels.
In Virtools there is something called "Render Priority": a per-object settings that can be adjusted in the hierarchy manager. It can help for these kind of situations. I was looking for something similar in Unity. The editor itself doesn't seem to have anything related but the shader framework, called ShaderLab, itself has.
So you have to download the built-in shader sources, and create a derivate. First I thought it's the SubShader RenderQueue-Tag, but that didn't had any visual impact. You have to adjust/bias the z/depth buffer value via the Offset state.
In my case I went through the "diffuse" shader and added an "Offset -1,-1" to any spot where RenderStates where set. Like you can see from the image above, it helped a lot. Unfortunately it's not perfect, I am still having tiny but annoying cracks here and there when driving or flying along the roads:
I guess I've no choice and adjust further the terrain heights manually to get rid of them…