Aye, Dux, that's the kind of thing I had in mind ideally, with 1 being 100% friction (representing bog or deep sand), .5 being average going, and 0 no friction at all, if a decimal number can be used. If it has to be binary bits, then mebbe at least two bits would be good, giving slippery, standard, hard going and impassable. If vehicles were then also given a "cross country" rating (0 for 4 wheeled vehicles, 1 for 6 wheeled, 2 for half tracks, 2 for fully tracked, for example) then the terrain value could be compared to the vehicle value so that 4 wheeled vehicles really couldnt cross impassable terrain, but fully tracked ones might just about manage it.
Juts had another thought... if small hillocks or ridges could be added to terrain as if they were buildings (so far as the software is concerned) that stopped shells without damage it would allow terrain builders to provide more interesting areas of terrain for ground combat. Tanks could go hull-down on the reverse of a small ridge, and hillocks and ridges could block lines of sight... something like the Normandy bocage might be put together... lots of possibilities.