It also occurred to me that you could even get away with making an actual, physical Z-level hill in the middle of the road.
Yeah it would block shots and such, but at the same time the road goes down the middle and would be a very clear and obvious division on the map. You could relegate that hill 'wall' to the edges of the map, put it in the middle, or whatever you'd like, but it's something to think about.
The limitation that we have with the Z axis doesn't mean we have to avoid using it. We can still try to be innovative. 
Such a "natural wall" by adding a hill to enforce the gameplay is often criticised by players.
I had such a "wall" at Westwall to divided the central map area into two sections and to allow tanks to retreat behind the wall. But it seems to me that this "natural shoot blockers" were often criticised.
So i guess when Beagle would add a hill between his two road sides a lot of player wont like it because of the shoot blocking effect
