Is there a law regarding trucks placing signs on the back with
“Not responsible for damage to your car, stay 200′ back”

Lately I have been noticing more trucks with this sort of sign on their lift gates.
Is it legal?
Does it release them of liability?
How did they get the idea that placing a sign makes it ok to drop dirt, rocks, and other matter from their trucks?

These are questions I am working on getting answers to. So stay tuned.

-Victor