Part of the problem--at least in my neck of the woods--is that some very heavy dump trucks seem to have worn out their mudflaps many eons ago. There is still a mudflap of sorts hanging there, but it frequently looks like it is about a foot shorter than it should be, as a result of wear and tear over the years. When these mudflaps are far too short, they do little to prevent rocks from being thrown by the rear tires.
In fact, on some of these trucks, I have actually seen the mudflaps riding on top of the rear tires, so that gives you some idea of just how worn-out they are. The result is as if there was no mudflap to prevent rocks from being thrown by those big tires. So, in some cases--or perhaps many cases--while the debris in question was thrown from the roadway, the equipment operator is actually responsible as a result of the defective equipment on his truck.
Regarding this problem, clearly the police need to start pulling these trucks over to ticket them for defective equipment. However, it is also clear that this is not a high priority for the police--hence the problem.
My solution, as was already suggested by others, is to follow these trucks at a safe distance, which is actually pretty far back. If I need to pass, I floor the gas and get past them as fast as I possibly can.