cops won't do shit unless they actually see stuff flying out. and even then, they still probably won't do shit.
it's a huge problem here in san antonio. my windshield needs to be replaced because of one of those douchebags that didn't cover his load. if your windshield gets hit, get the truck's license plate number, and call up the company that owns the truck. they are responsible for any damages caused by their negligence. those "stay back 200 feet" signs are total bullshit.
oh, and one of our local news stations did one of those "expose"type things. they spoke to several truck drivers who had't covered their loads and were dropping rocks all over the highway. the truck drivers' response was "i don't really care", or "it takes too long to cover my load, so i don't do it".