Are our Slants "Unsafe"?
Nope. When people act like selfish little cryabies ("I CAN DO WHATEVER I WANT WITH MY CAR! YOU CAN'T STOP ME! YOU CAN'T MAKE ME! NOOOOOOOOOOOOOOOO!") in a sparsely-populated rural setting with not many people or cars or traffic around, it's not of particularly great consequence. As population and traffic density increases, everyone's decisions and everyone's behavior affects everyone else more and more and more. Sooner than later, the selfish little crybabies begin to pose a serious hazard to those around them, and if they can't be persuaded to grow the hell up and behave like adults, laws get written to constrain the harm and hazard they can threaten others with.
Think about it for a minute: One person eating, farting, and crapping in the forest doesn't do much of any harm to anything. A hundred people eating, farting, and crapping in that forest is very unpleasant. A thousand is a health hazard. Ten thousand and the forest is trashed and everybody's very sick. It works the same whether we're talking about cars not adequately equipped to participate in modern traffic or any kind of pollution or pretty much anything else.
But srsly, dØØd: Relax. Take a breath. No big, bad guvmint man's going to come take away your car or say you can't drive it. People have been babbling about the "nanny state" forcing old cars off American roads for as long as I've been paying attention (well over 20 years now) and it hasn't yet happened. Every single day, there are fewer and fewer old cars on the road, which means every single day it's a lower and lower regulatory priority, which means it's not going to happen.