When we are teaching our students about literacy in the ancient world, we spend quite a lot of time explaining that public writing in the ancient world is not necessarily meant to be read, and so a town full of public notices isn't necessarily a town with a high rate of literacy. This usually leads onto to some discussion of the different functions of the notices we see around us, and their different levels of efficacy. Frankly, the little notice above (I cant remember where I found it) is much more likely to keep me off the plants than some humourless directive just to keep off.
And although it is a little more formal, this notice about (not) bringing drinks into my own college library manages to strike a pretty good note. I mean to say, I wouldn't take a can of coke into the library after I'd seen that.
Others just bring out the counter-suggestible in me. To be honest, when someone tries to tell me how to wash my hands (right down to number of ml of liquid soap), and insists that I turn the tap off with a paper towel, puts me off washing my hands at all. And I dont think that I am the only one. (It wasnt in a hospital, by the way!)
In other cases though, the point of the notice seems to be something quite different
Is there anyone on earth with a milk intolerance who does not know that cheese contain the vile substance or needs the logo to tell them what milk looks like (and this, I confess, in an institution of higher education). This smacks to me of a "dont try to sue us if you get ill" kind of notice. Much like the following, though I can't for the life of me remember what dish this was next too (the warnings rather put one off . . . ).
Just occasionally the joke seems to take over, probably at the cost of the message. This little reminder about passwords being like socks (from the European Commission in Brussels I think) made me laugh, but I suspect would have no effect on the regularity I would change my password!
Though this one (from my college) takes the biscuit for incomprehensibility to the outsiders, while including vital info for the insider.