People really need to watch some crash videos with people without a seatbelt, you become a fucking projectile, I have seen one where two women are sitting in the back one without a belt and she slams really fucking hard into the other one that had a seatbelt, as in you can hear the head on head on contact.

For frontal crashes if someone is sitting behind you without a seat belt, they will literally crush you to death above a certain speed.