I never said I've never come across them; I have. If you look closely at the photos on Debito's site, you'll see that one of them is mine.
That being said, though, if we included all the countless thousands of places I've seen that did not have such signs and do the math with the mere handful of places I've seen that do have them....the ratio works out as I said: how close it is to zero depends on to how many decimal places you figure it.