Same thing happened to me on I 40 between Raleigh and Greensboro in the 80s.
Mine came from the flat bed that was 1/4 mile in front of me. I saw the whole thing, it was like slow motion, the plywood was just floating, then....WHAM, right into the windshield! Sounded like a gun went off.
I pulled off and 2 other cars stopped with me to see if i was ok.
The only evidence that i'd been hit was a streak of blue paint on the windshield they use to mark the grade of the plywood. The paint came off with a little rubbing.
I spent the first half of my career on the hwy and have hit lots of stuff, very dangerous.