This is actually a great idea. Too bad if it's enjinbinatation.
I wonder if random bits of wood or rocks or debris lying around in these fields could be coded like the fences, but only allow halftracks, tanks, and citroens through unhindered. The trucks could still run full speed, but there would be a high chance of hitting one of these and rolling/bumping around and generally making it difficult to drive off road.
They could be pretty simple low poly objects.
Or long, invisible, speedbump-like objects.
Infantry could possibly walk over unhindered as well for gameplay.