Low walls can be crossed by tanks and infantry, hedges and fences can be crossed by anything I think. One good tip is that whenever you're driving over something like a hedge or wall the slower you go the less damage it will do to the tracks\wheels. I always break through stuff with slow, a lot of the time it doesn't do any damage. That also applies to driving through some types of rough terrain. Going quick through heavy brush for example will cause much more damage than going slow.