I realize you were going to a Marriott, but don't think this is really related to Marriott per se. I'm sure the company that sells the shuttle sells them to multiple hotels, etc.

Also, I don't usually think about drivers rear-ending others when I'm in a shuttle & it never would have occurred to me to lean forward to see if I could hit my head, but realize that's a concern you might have. I'd actually be more concerned if the shuttle had no seat belts at all.

I was in Charlotte several years ago when they had an ice storm. Shuttle driver hit some ice, went across the median (not a highway) & hit a small tree. Some folk got thrown forward & the one that got injured did so because of no seat belt or not wearing it (Hilton chain). BTW - this shuttle wasn't the only one that didn't do well on the road. Charlotte doesn't normally get ice storms, so they didn't have salt etc on the roads.

If you're worried about the safety of shuttles, my recommendation is to take a cab - and put on your seat belt in the cab.

Cheers.