People are missing the point of seatbelts, and why other people should be interested in whether people are wearing them or not.
Seatbelts and helmets are not analogous, because of the potential range of injury. If someone has an accident without a helmet, they will injure themself. If someone has an accident with no seatbelt, they can potentially kill other people - even people not in the car.
When the law changed in the UK they had some very graphic adverts about back seat passengers crushing those in the front, or going from the back seat through the front window, damaging everyone on the way - including people in front of the car originally protected from the accident by railings, bollards, etc.
This post ^^ uses the example of IF you have an accident AND are not wearing a seatbelt. Which isn't the same as "not wearing a seatbelt can cause you to lose control of your car".
I'll admit, I don't agree that motorcycles are any different. If a motorcyclist has an accident without a helmet, they can potentially kill other people as well. Say, if he was wearing a helmet, he might have been conscious and able to get himself out of the road before another car came along, but if he wasn't wearing a helmet, might not have been conscious, thus stayed in the road, and had an unsuspecting car added to the collision by swerving to avoid hitting him, and into oncoming traffic, the end result (possibly, of course) being a head on collision with another vehicle, killing some if not all occupants of the vehicle. So, helmets are just as important as seatbelts, in my opinion.
Bull flops. Seat belt laws prohibit your doing something stupid that endangers the lives of everyone within a hundred feet of you.
There is no equivalency to prohibiting your eating junk food. More like prohibiting your throwing the garbage out in the alley to attract rats.
If you and your passengers are not secured to your seats, you are not in control of your vehicle and have a far greater chance of losing control in an otherwise managable accident, thus of harming somebody with a more-than-room-temp IQ.
Although it may be better for the species to let the kind of people who do not wear seat belts to eliminate themselves from the gene pool, it is not a good idea to allow them to take out more geneticly fit people along with them. That's why we have traffic laws in the first place.
This post ^^ is not using secondary injury solely to justify the law, but is instead using it to poke holes in the analogy it's responding to, explaining why not wearing seatbelts is not the same as failing to eat healthy, as eating unhealthy harms no one but the person doing the eating, whereas failing to use a seatbelt can potentially harm an innocent bystander (say, by your body hurtling through the windshield and hitting them).
It also is commenting about IF you
and your passengers are not wearing seatbelts, then you have a "far greater chance" of losing control of your vehicle. This is patently true, as anyone who's had escape-artist toddlers in the back seat of their car can attest to (how in the heck they get those safety belts off their car seats, I will never know, but damn, kiddos give Houdini a run for his money).
It is also patently true that traffic laws are, by and large, implemented to avoid collateral damage from accidents, not to prohibit idiots from killing themselves. Drunk driving, for example, wouldn't be quite so objectionable if only the drunken fool got hurt. Speeding, also, not a big deal if only the moron going 150mph down a 25mph road got hurt. But when you're talking about motor vehicles, it's never ONLY the person screwing up who gets hurt. Even if there's no actual other person physically injured, there is property damage, cost to the government for emergency services being dispatched, the risk of the emergency service personnel being injured, harm to the economy by losing part of the labor force and having to take over support of the family (potentially) of the hurt/injured imbecile... And these "secondary injuries" are not immediately quantifiable as we're talking about ripples and waves of consequences.
Heck, these ripples of "secondary injury" are pretty much the philosophy behind most public safety laws.
Are there concrete numbers? Perhaps. The NTSA compiles huge reports on the reasons behind, and the results of, traffic accidents. But the people talking about secondary injury are talking about how lack of seatbelt use risks
potential secondary harm. Lots of "if" and "could" and "chance of" in their posts. They're not making concrete claims. *shrugs*