I suspect, as Harry implied, it's more about internal frame flex or from another POV how rigid the internal frame might be. Given the corners cut by device makers these days I have no doubt the frames are not near the level of a laptop.
No doubt the seatbelt could well be the culprit. Honestly I can't imagine carrying anything other than an old PDA or my PB360 in a pocket let alone strap it down with a seatbelt while it's in my pocket.
I've owned five dedicated ebook readers, all of them with eink brand screens. They all have traveled constantly. Never a hint of a broken screen among any of them from the K1, K2, K2i, DXG and PB360. But then again I make a conscious effort to not put it in a position where they is anything torquing the device. And every one of them has had a quality cover. All of my Kindles have sported an A-frame style cover from M-Edge and my PB360 has an integrated cover of it's own though there are limits to the pressure it can endure.
I carry my readers everywhere and have expected a broken screen along the way but nothing, so far anyway.
knock-on-wood!!