I have thought about something like that for a long time too. So you are not the only stupid one here Gareth. My proposition is to do it in Photoshop. Create the grayscale image for the rolling hills and then do an extra layer (not lawyer) for the road so that it just slightly modulates the underlying gray levels. Like using "multiply" or something like that instead of "normal" for the road layer. Could do. Another option might be to do a grayscale image for the road "as usual". Then do a terrain in Vue for the hills and overlay the road grayscale in the terrain editor. And play with the percentage of overlay to (hopefully) make it follow the hills. Just my 2 cents, hope someone else has a more clever trick. Could be a dirty trick of course too, Gareth asked for a dirty road anyway.

One day your ship comes in - but you're at the airport.