Additionally (IANAL), if someone knows about something dangerous and does not reasonably mitigate it, it goes from negligence to gross negligence if something happens. Having a sign indicates that someone knows about it.
Also, a sign can say anything like "We are not responsible...", but that doesn't mean that's true. (Take a picture of such sign if anything happens, because it's evidence that will most likely help you in court, again IANAL.)
