1. What if you hit a barrier? They are literally designed so that a person behind the barrier does not get hit, but normally they are lower than the car, so you would still hit them.
2. Proving my point that it is not a victimless crime.
3. What is this lower hanging fruit? Putting on a seatbelt seems very simple.
4. No, this is not your body as a projectile hitting someone, but you being unrestrained prevents you from staying seated and so can't brake or steer effectively. This can even happen even when do not hit something, but just hydroplane or skid.
1. Yeah that's a good example when it could happen. I expect that is rare enough that it would reasonably fall to insurance rather than regulation. We simply can't regulate every small chance event that could impact others.
2. Victimless here matters in context of regulation. It seems reasonable to consider someone emotionally harmed is a victim, though its important to decide whether emotional harm felt by one is a direct action caused by the other. For example, if someone emotionally responds to seeing my dead body I didn't directly force that reaction on them and I wouldn't say there is direct responsibility for it.
3. We aren't talking about the act of wearing seat belts, everyone should choose to because it is easy. We're talking about regulation and government authority. Regulating sugary drinks, for example, would almost certainly be more impactful.
4. Brakes aren't the problem if the vehicle stopped quickly enough to make me a projectile.
And to be clear, I to wear a seat belt and want everyone to choose to. I just don't want a government to have the authority to require it and fine us if we don't do it.