NOTE: We can debate whether it's 80/20, 90/10, or 70/30... but the exact amount is a distraction. We know that most of the time, seatbelts do decrease injury severity... and in some cases, it magnifies it. The exact amount is irrelevant, the question is whether government should punish people for living their lives in ways that endanger no one but themselves. But as a Libertarian leaning person, I think you can guess my answer to that.
If you know math, you understand the differences between averages and individuals. While seatbelt laws do reduce injuries or fatalities by around 8-9% (depending on who is measuring and how), that's not a huge amount -- and that's average. What you have to remember is when you look into cases, there are many who will die or be hurt worse by seatbelt laws (or wearing seatbelts) and a few more that will be saved or hurt less. In average, everyone wearing a seatbelt, is theoretically a net win... which is why I wear mine. But in individual cases, forcing everyone to wear a seatbelt will kill or harm some people. (Common deaths are caused by being unable to detach a seatbelt in a fire, rollover, or in the water -- which can kill the wearer).
More than that, while the theory is that it'll save lives, the practice is, we have no evidence of it.
If you examine the highway fatality statistics, which have been kept since at least 1950, you'll find no sign of influence from state seat belt law, just the inexorable decline in fatalities per mile we've seen ever since we started keeping those records. Of course, we measure them at fatalities per million miles driven now. Seat belts probably save lives, but so do radial tires, which keep cars on the road far better than did bias ply tires. There's a lot of nuances (like Speed doesn't kill, but the differential between speeds does). The safest practical driving speed is about 2 mph than is the flow of traffic, regardless of how slow or fast that traffic flows -- and with that in mind, the States decided to put high speed HOV lanes, next to low speed parking lots, and got the expected increases in accidents and fatalities as a result. While fining us for not wearing seatbelts. Gee thanks.
I have no problem with people making either choice, on their own. Like I said, I lean towards wearing mine. But forcing others to make the choice you think is better for them, even when it will cause some of their deaths, is actually a pretty shithole move. Education, influence, encouragement, all good things. Forcing them to do something that will kill some of them, means YOU own the consequences of that action (instead of them owning it). And while we can do the gruesome calculus of saying, "these three will live, but that one will die because of that policy: oh well, sucks to be him" is a dark, dreary society. Basically, you're not just deciding the Trolly Problem for yourself -- you're deciding to force your answer on to everyone else. And that's a pretty dick move, if you ask me.
I'd rather let them make their own choices in life, and NOT own the consequences and have their blood on my hands, than be the big-government Nanny-state person that kills others for my causes. But that's why I'm not a progressive.