Danger: Warning overload

At the park today there was a sign with various warnings and rules on: No dogs, no glass, no litter. And the one that said: Say no to strangers.
Whilst pondering how many of the children who use the play area can actually read, and of those how many would read that sign, I wandered home and said hello to three strangers on the way. Little man was watching and listening over my shoulder.
When it comes to warnings about child safety I tend to think that many of the people who need reminding probably don't read signs and posters and leaflets. And compulsive readers like me don't need reminding.
On the subject of signs, I am intrigued as to who put up the one pictured on the corner opposite my house.