Physics for vehicles (and infantry as well) have to be reviewed. Looks like every kind of terrain is exactly the same to me apart from sound effects. Have you ever been driving offroad or simply hiking in the hills ? Cannot be done at fullspeed without risk huh ? Please check what Spintire devs (www.spintire.com) made about vehicle physics. Download and play the demo or youtube it. This is how vehicles behave in offroad conditions. Not mentionning the actual weight of military vehicles, armoured or not, not mentionning the bulk of one infantryman even in good shape and trained... Thats why man invented trails and roads. Climbing and going downhill offroad slopes in a 4 tons armoured car is tricky and slow. Kinda situation to be avoided in combat when tricky and slow mean vulnerable. This is why infantry fights afoot. This is why recon is important etc.
How to do that ? Adapting speed on terrain, maybe taking damage (vehicle and crew) for overspeeding downhill...
What is the benefit ? Terrain will finally be taken in consideration when planning an attack or a patrol. If u know it's gonna take ages to cross this openfield at the risk of being spotted and destroyed you'll take another option.