It is fascinating how two pieces of wood stuck together with a rudimentary set of painted letters on it is a sufficient deterrent to humans from venturing further up a path; this is a basic tenet of the relationship between the human cognitive process and semiotics.
The sign could have been planted there by a group of pranksters, and you wouldn't know, but the psychological conditioning you have been brought up with will force you to instinctively obey; in some cases, it will be obeyed whether or not it makes sense or could potentially endanger a person.
Also noticeable is that a sign reinforcing unwritten rules will only foster resentment instead of helping the situation (ex: A "no shouting" sign in a workplace). This is an example of authority overreaching itself - people like to be told what to do, just not what they already know or currently accept by default.
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