When I used to be driven from Arnhem to Oberhausen (ca. 199x) it's been very obvious when you've crossed the border to .de, not even because of the checkpoints, which still existed then, but because of the noises the car made when it went over the gaps between the concrete plates which probably were the same as in 194x, or so, when they Autobahn has been built.

Very annoying when stoned!