Except we don't throw people in jail for not following seat belt laws, we don't "point a gun to their head" as jsem implied. Basically, if you're pulled over for doing something else illegal with your car, you might get an extra fine.
Ok, gun to the head is a bit over the top. But there is still an initiation of force. Because if you did nothing wrong (not looking at the law here), did not violate someone's rights, and some one forces you to pull over - that's coercion. What if you don't want to pay they fine? I'm not sure what the law says - but whatever the case there is still a threat of violence from the authorities.
But anyway, if one would see the government as a private owner of the roads, and the roads weren't "public" - it'd be up to the owner of the road to determine if there should be a rule enforcing the use of seat belts. Perhaps on a private road, the owners would highly enforce the use of seat belts for the higher chance of survival so that there can be agreements made by the two car owners and their insurance companies in case of accidents - or something of that nature. Or maybe the road owners whats to enforce it for the general safety of the drivers.
The only problem with this view is that we are coerced into paying for the government roads, which is a violation of the non-aggression principle in the first place.