Its the same effect shown in urban behaviour where a messier neighbourhood leads to worse public conduct. If you see people trampling all over a patch of grass with a keep off the grass sign obviously your more likely to ignore the sign.

Here the fence being flattened shows the sign and fence are safe to ignore because somebody else probably has already

I think the automatic response is to say “If there were any true danger, someone would have bothered to repair the fence” and therefore to ignore both.

I think the meaning would be very different if the fence had been trampled going the other direction…

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