An operator of a truck is responsible for their load. The load is required to be properly secured (i.e. you can't have debris flying out of it - that's just plain dangerous). No amount of signs circumvent that matter of law. A driver of a rig that had its load shift was convicted (or pled guilty) to criminal charges resulting from an accident that killed people locally. His responsibility was due to the fact that he didn't properly secure his load and nothing else. It was a big deal on the news when it happened. Big fiery wreck on the interstate with cars crashing into the back of it and all.
Of course, vehicle damage is a civil law issue, but I doubt civil law absolves an operator of the responsibility for their load.
