I think she probably saw the result of what happens when you're going 60 miles an hour and your arm hits something standing still or worse something going the same speed in the other direction. Not pretty and the reason I was never allowed to put my arm out of the window as a child. My mum knew someone that I think shattered the bones of their arm that way (can't remember exactly if it was just shattered or they lost the arm, I just remember her freaking out if I tried to do it) and was extremely paranoid I would do the same. I also know someone that cut off their own hand with a chainsaw (and his son cut off his own finger with a table saw that he'd idiotically taken the guard off of) so I'm super paranoid about them myself. No one in my family works in healthcare we just seem to know/be related to idiots.

The car rolls and the arm suffers a degloving injury or gets crushed.