During my first attempt at college I had a Dynamics class one summer. The professor did side work for auto insurance companies on wreck investigations.
We calculated the forced to which the body, specifically the skull, when it impacts something, such as the windshield header as the unbelted body goes over the air bag and strikes against it. It wasn't pretty. Something like 1" for a 35 mph crash I believe. That is 1" your skull goes into your brain.
Impact forces are wicked. If you have a temp memory lapse, just hit your finger with a hammer and see for yourself.
This is why I just cannot ride my motorcycle without a helmet.
Hell, I have trouble driving 1 block from the mailbox without a seat belt. I feel like I might fall out of the seat...lol.