Public places and release forms?

[the7ferret]

When i go to certian events I see posters, placards, and other signs that say "filming in progress.. blah blah blah if you dont want to be filmed dont enter blah blah blah..." In public places and domain can you use that as a subistute for release forms? ect ect.

[PhotosGuy]

It might keep someone from thinking of suing, but wouldn't CYA as a release form would.