When you have your roads system just as you need itClick on the Modify Terrain to match road To updated the terrain.If you are not happy with the terrain you can press on Restore previous terrain to undo the modification.
Terrain Options
Plateau: The amount to level off on both side of the road
Slope: The amount to smooth out to match the existing terrain
Flatten Terrain: Flatten the terrain before modifying
Details-Off Set: Off set the height of the terrain from the road (reduces the chances of the terrain clipping the road)
Details-Divider: The higher the details on the terrain, the higher this number will need to be.