The energy has to go somewhere. So if you are not wearing a seatbelt and the car flings you out like a projectile, the car itself will have slightly less kinetic energy, being slightly less dangerous.

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?

Not everyone riding as a passenger in a car has choice in these situations, namely children.

People are already allowed to eg feed their children unhealthy food etc. That's usually not classified as an externality.

Putting yourself and a child in a flying mechanical deathtrap and you're not wearing is an unnecessary risk to their life in the same way that leaving guns loaded and unlocked in a house is. These are differences in the magnitude of risk and a large enough difference in magnitude becomes a difference in kind.

That's mostly an argument for the law requiring children to be buckled in at most, perhaps.

> the car itself will have slightly less kinetic energy, being slightly less dangerous.

Extremely slightly less. What, 170lbs compared to 4,000lbs+? Is 4% weight reduction after the collision starts, minus the energy imparted to the seats and other passengers and the windshield and what not, really going to make much impact to the overall collision calculus here?

Yes, this won't have much of an impact. But neither is there much of an _additional_ danger to other people from you flying around in an accident.

There is, because people can and do go through glass. Sure, most of the time this means that their brains are just spread on the highway like nutella. But they could also go through YOUR windshield.

I'm trying to read your comments charitably and made in good faith but I'll admit it is proving to be a challenge here.

In the UK they ran a campaign to raise awareness that during an accident a passenger riding in the back can injure the passenger in front if they don't wear a seatbelt. That seems like a serious externality to me. I don't know if you just hate seatbelts or love arguing but there are absolutely reasons that people should wear seatbelts and I'm glad most people have accepted that the inconvenience of wearing them is pretty negligible now.

https://m.youtube.com/watch?v=mKHY69AFstE&pp=ygUeVWsgc2VhdGJ...

https://www.reddit.com/r/YouShouldKnow/comments/359am5/ysk_i...

It's not an externality if it's in the same car.

If you are a driver, and you want people in the back to wear a seatbelt, just ask them to wear a seatbelt. Duh.

> I don't know if you just hate seatbelts [...]

Why would I hate seatbelts? I wear them all the time, whenever I have to take a car. I just don't buy the usual justification for forcing other people to wear seatbelts (or have AM radios..)

> [...] and I'm glad most people have accepted that the inconvenience of wearing them is pretty negligible now.

I'm all for people wearing seatbelts, too. Just like I'm in favour of people eating their vegetables and flossing their teeth. Voluntarily.