The way dump trucks are made rocks cannot just come flying out of the back of them. At least not any good size stone like a size 8 or 53. Have you ever seen a piece or rip-rap come flying out of the back of a truck? Smaller stone like 11's,12's and 13's will blow out of a dump truck just like sand or dirt will. Alot of times rocks are flipped up by the tires on a truck or the tailgate is leaking. One of the most common causes is when you get loaded at the pit you sometimes get excess stone on you drip pan. Some drivers are too lazy to get out and clean them and when they start down the road those rocks are going to go somewhere. When I drove a dump truck I never had anyone complain about flying rocks until I put new tires on. As soon as I did I had at least 3 call ins about a leaky load. Seems when I would pull into the pit my new tires would get rocks stuck between the tread and then when I would leave and hit a great enough speed, out come the rocks. So what do we learn from all of this? Stay the hell away from ALL BIG TRUCKS EVEN SEMIS WITH TRAILERS! Ever been behind a big truck that blows a cap? how bout beside one? Always stay well back behind any truck and if you want to pass do it and get it done. Don't ever ride beside a semi going down the highway for very long.I realize you guys are *****ing about truck drivers but until you have been there and done that you have no idea the kinda things people in "four wheelers" will do not thinking that a big truck is just 60-80 thousands pounds of metal that will kill you if you get in it's path.