When I was in Driver's Ed, some guy came to the classroom - I forget what he was there for, some maintenance guy or something random like that. He might have been there to talk about motorcycles. He started telling that urban myth about how he knew a couple who were killed when their car crashed and the seatbelts they were wearing decapitated them. The actual teacher's face was completely WTF. You could tell he wanted to tell the dude to get out. 16 year olds find enough ways to kill themselves. Don't tell them to stop wearing their seatbelts!
Don't know about the decapitation angle, but you know as well as I do that shoulder straps on seat belts are NOT designed with short people in mind. I could see the damn thing breaking my neck...if I didn't keep a bungie cord around it just above the buckle. That keeps the shoulder strap across my shoulder & chest, not across my neck.