tmurray
SAWHORSE
Unless it's part of an unsecured load (example: a 2x4 falling out of a stack in the bed of a pickup, or a ladder falling off a ladder rack), I doubt any court would rule the owner of the truck to be at fault. Your insurance is going to pay for the rock that cracks your windshield, not his.
Anecdote: In another life, I (briefly) drove an over-the-road truck for a living. I was on I-20 in Georgia in the wintertime and a sheet of ice half as big as the trailer I was pulling flew off the top and smacked a lady in an SUV square in the windshield. Hit her hard enough that it bent the A pillars. Thankfully she didn't crash and wasn't hurt, and just pulled over. I saw it fly off the truck in the mirror and pulled over as well. Called the police, did a report, the whole deal. Cop did not write me a ticket. Her insurance paid for her SUV and nothing went on my record.
Local police here have been writing tickets for "unsecured load" for snow/ice buildup that flies off vehicles and strikes another. Now anytime I've heard of it, it has always been someone in an SUV/van where the owner was just too lazy to clean off the top of their vehicles. We have catwalks that commercial trucks can pull under to clean off the tops of the vehicles and trailers. Most are very good about doing it.