I think, at last count, 40 states have some sort of equestrian liability
law. We worked for years here in Iowa, finally got our's passed in the
1997 legislative session. These laws don't cover everything, but do tend
to reduce the number of "frivolous" lawsuits, and posting signs such as
the one you described helps to make the public aware that they must take
responsibility for themselves when they choose to participate in
horse-related activities.
Ruth