this was used here in Cleveland during Spiderman 3 shoot, they had the street blocked off and they posted this all over the area, and yes, it is legal and it is meant for a crowd when there are too many people to ask for signed waiver.
The signage was placed at the begining of the streets and also it is also 'loud speaker' throughout a few times, now if people claimed they didn't know there is already public display of signage so they can't argue in court that it wasn't there. Basically, once this is visible, and everyone (or majority of people see it in a visible spot) they will have to comply with what is said there, just like those public parking lots, where you see signs that said "park on your risk, if car is broken in, we are not responsible, etc'. Same premises
I've used similar during the shoot for The Rapture, in every building or outdoor we shot at, we have sign posted at the entrance area and also have PA footing to make sure no 'strangers' are in the set. Furthermore with outdoor shoots, we notified the police and they've also sent out an officer to be with us during shoot (especially those days that we have a river partially blocked off and also a street blocked off).
Johnny