Here are 2 examples on how to project a road on a terrain, starting from a flat road and a bumpy terrain.
Tip on how to project a road on a terrain and keep its 3D thickness intact. First, you'll see how the conform compound object cannot succeed in this operation. Then, you'll see how to use the skin wrap modifier to keep the thickness of the road as it follows the terrain.
Tip on how to use the morpher modifier and the previous technique in order to use any terrain shape and match a 3D flat road on top of it.