They are not responsible for any objects kicked up by their tires.
In Texas, they *ARE* responsible for anything that falls off their truck .. it falls under the "properly secured load" rule. Infact, Texas-DOT recently passed rulling requring all sand and gravel trucks be tarped when loaded. Boy did the rock and gravel guys hate that.
Anyway .. I really think those "Stak back 250', not responsible" signs are really neat .. cause every dump truck I've ever seen has had no problems entering my lane alot closer to me than 250'. If one day one of them drops something and damages my car .. I can't wait for them to say I shoulda been back 250' I'll be like .. yeah .. I was .. YOUR DRIVER PULLED IN FRONT OF ME, and since you realize that a 250' distance is necessary, your responsible for not following your own rules.
Oh well .. one can fantasize anyway ..
back on point .. if it fell off, they are responsible in most states YMMV.