Seatbelts protect other passengers in the car.

People are terrible at assessing risk and they probably need a gentle legislative push for some things, like seatbelts.

Alright. Often there's only one person in the car though, and what I said applies to other things too.

But the accidents are not usually single car incidents.

So, you are then leaving the driver of the other car having been in a fatal accident, because you did not want to wear a seatbelt.

If you want to drive on a private road without a seatbelt, drunk, whatever, that is entirely your choice. If you are likely to interact with other road users, then your choices impact others. (Note: your choices on a private road impact your family etc, but, you do not risk leaving someone feeling guilty because they had a car crash which killed someone)

We're not talking about other things. We're talking about seatbelts.

I already acknowledged your point. You could do the same for mine.

It's a bit of a stretch to say that seatbelt tickets lead to tyranny--driving's mostly a privilege not a right. Seatbelts can affect other people due to an inability to control the vehicle after a collision, more likely use of emergency resources, etc. I generally agree with the sentiment about nanny states, though

Didn't wear a seatbelt and headbutted the person in front, snapping their neck. Sucks to be them, I guess. But at least you stuck it to the man.

Does there exist a single incident like this?

Just playing devils advocate, but I know people who won't drive a car without making sure everyone has their seat belt on.

Alright, you got me. Now imagine you being the only person in the car and try again.

Seat belts help you stay in control of the car in the event of a collision. If you're the only person on the road, you might have a point.