The idea is for the sign to describe how it in fact works, not necessarily how they'd like it to work. (A sufficiently detailed sign might explain the distinction, potentially allowed for coordinated punishment of defectors.) That's why I mentioned the bit about "the goodness of their hearts". It would probably require a law because of the problem of people stating outright how something "really" works.

(I've been to the Hauptbahnhoffs btw -- "links gehen, rechts stehen" is the phrase, right?)

Introducing a formal rule set when people are used to following informal rules is hard.

I agree -- so the idea instead is to have a sign that can quickly teach people this informal system, since it may be so hard for a newcomer to infer it.