I would say that roads are terrain, but they do not cancel the terrain that they overlay. Hence you can ambush on a road, and you can plash across it. I also think you could pay for a gate in the plashing if you want your ambushing troops to emerge at road speed (this would also apply to an ambush on a road in a fortified BUA).
Plashing and barricades are not elements of mounted, foot or naval, so are not affected by the limit of 8 elements of these.