At least in California, its illegal for anything to fall from a vehicle except water and bird feathers so not sure how that sign help them.
They do absolutely nothing to remove liability from the truck driver/company. If a rock falls from their truck and cracks your windshield, they absolutely are responsible for any damages.
Rather, their sole value is to convince drivers that the trucking companies aren't at fault, so that drivers whose vehicles are damaged from falling rocks erroneously elect not to press charges or pursue damages.
If you are tailgating directly behind a rock truck with a big sign "stay back 200 feet" for an extended period of time, or end up right behind the truck because you're in a big hurry, or because you thought you could squeeze through an empty lane, a good lawyer could absolutely argue, successfully, that you are at least halfway responsible for the damage, if not 100%.
> the law is really clear
what law are you quoting? in what city/state/country?
I certainly agree that in many cases a rock truck causes damage to entirely innocent drivers who happened to get in the vicinity of spilled rocks without ever intending to (for example if the rock truck passes them, or at intersections, etc.
However -- you said "they absolutely are responsible" and I'm saying, no, it depends. Rock trucks are annoying and dangerous but are clearly necessary for cities to build roads and other infrastructure. Unfortunately, it seems impossible to fully, absolutely secure a rock truck. If a rock truck company came to court prepared with evidence that it had followed (or exceeded) every safety and regulatory procedure, and perhaps that its accident rate is lower than industry average, and further, that the "victim" was tailgating right behind the rock truck (probably in attempt to pass) despite a prominent "stay back 200 feet" sign, even a small-claims judge might say, it's half-and-half, or -- especially if the truck had video of the other driver performing a dangerous maneuver -- that it's actually the driver's fault and therefore no liability from the rock truck company.
> Really all they will do, is make it so painful for you to get in front of a judge, that you give up.
because a dangerous driver will realize that their case is extremely weak due to failing to follow the 200-ft sign. mission accomplished.