[narwan]

Many roads, especially highways, have viaducts that can be easily blown up. Where off ramps from highways don't use viaducts they are usually on a some what elevated level. You don't have to drill through the tarmac if you can just go through the side through the packed earth. Highways are quick, but hard to get on and off, especially if the off-ramps are taken out. Units on it will be sitting ducks with very little cover or escape options.
Then there's for example the clever use of mines and booby traps to block routes around choke points, digging of deep trenches to block trucks, blowing up (high) buildings next to roads to block them, and then we're not even mentioning the use of nuclear demolitions or persistent chemical agents. Soviet AFV's may be protected from their nasty lingering effects, their trucks aren't.
There are so many ways to block the handful of vital roads from east germany into the west.
For me it wasn't until I started travelling between east and western europe in the early 90's and saw with my own eyes that I realised just how little infrastructure there really was connecting east and west. And not just across the border but also behind the border on the eastern side. Even without NATO harrassing them getting troops and supplies across the border would be a logistical nightmare.

Narwan