Fitting roads to 3D terrain

[DanS]

I am trying to find a quick and easy way of displaying roads/pathways etc on a 3D terrain formed from an interpolated surface. I have been duplicating my basic surface and then intersecting an extrusion the shape of my road with it. I was then hoping to raise the intersection say 50mm or so above the original surface so that it would then display when rendered. The intersection though does not seem to closely follow the profile of the terrain and I am having to raise it by half a metre before it displays OK. Any ideas??