In any case, even if you discount energy conservation, the extra danger to other people from you becoming a projectile is likely tiny. You can run some cost benefit analyses, and I'm pretty sure you'll come to the conclusion that a Pigouvian tax of something like a dollar a year is enough to offset this.

> One persons dumb choice to not wear a seatbelt can have potentially fatal consequences to others riding in the car who are not responsible for that decision.

Are you talking about people in the same car as the guy not wearing a seatbelt? Then you can exactly identify the other parties, so the transaction costs for Coasean bargaining are very low. See https://en.wikipedia.org/wiki/Coase_theorem Basically, if you think it's dangerous to ride along with people who don't wear a seatbelt, then don't ride along with people who don't wear a seatbelt.

As a pedestrian you can't opt out of what drivers are doing. That's a real exernality. But as a fellow passenger, you know exactly who else is in the car, and you can refuse to ride with them.

If I understand your suggestion right, it's like marrying someone with bad breath, and then asking the government to make a law to make your spouse brush their teeth?