Not responsible for damages...really?

I found myself closely behind a dump truck the other day when I changed lanes. The dump truck had a written warning, "Stay back 200 ft; not responsible for damages." Really? Is the dump truck company really not liable for the damages that their trucks cause to other people's property? There is no valid contract between me and the dump track company in which I've agreed not to hold them liabable for damages to my property. Here is an example to illustrate the absurdity of the dump truck warning. Let's replace the dump truck with a SUV with children inside. The warning on the back of the SUV is the same. The children throw rocks out of the rear windows. The rocks hit my car and shatter my windshield. Do I have a valid claim against the owners of the SUV, or does their warning eliminate their liability for the damages that their kids inflicted on my car? Of course the SUV owner's are liable for the damages; the dump truck company is liable for the damages they cause (regardless of what the warning says).