Years ago one of those triangle 'men at work' signs (stand and all..!) got flicked up by the car in front (M6) at about 70 ish. I watched in slow motion as it flew through the air and smacked into the front of my 205 GTI. Other than braking sharply there was nothing I could have done to avoid it (and yes I was a reasonable distance behind the car in front)..!
Luckily it was just before the Police stop/station slip (you'll know what I mean if you know the M6 around B'ham) so I pull off to a/ check how much of the front end was left and b/ report it so I can hopefully claim off the bloody idiots who hadn't secured the thing.
Guess what, the very helpful policeman couldn't have given a toss..! He said that they would send someone out to get it removed, but even though road signs should have an ID number on the back to say who it belonged to, they'd just say it wasn't theirs and there would be very little else I could do. Couldn't even be bothered to look at the damage to the car. He did however have time to give me a producer as it was still classed as a road traffic incident..!??
Luckily the damage was surprisingly light (scratches on front wing and a broken door strip) which given the bloody size of this thing and the bang it made was unbelievable. So a new door strip some T-cut and a bit of touch up sorted it out.
Therefore, IMHO, something like a brick in the road leaves you without a leg to stand on (unless you see it drop off a vehicle and take the number, just don't expect much help from the old bill)..!