> Which is a good idea because if such a safety feature remains optional, many people won't opt for it. Making them mandatory saves lives.
On the other hand, making them mandatory makes cars more expensive, which means (especially during a shortage) that some can't afford them at all while the rest are forced to do without other things to compensate; either way it reduces their qualify of life. But of course the government knows better than those actually affected by these laws where the ideal balance lies between safety and QoL.
On the other other hand, you can easily use the same reasoning in favor of mandating safety features:
Not making them mandatory means they remain expensive extras, more expensive than if required on all cars. That means second hand cars with these features will be significantly more expensive than those without. Which means that some can never afford these features, while others have to do without other things to compensate. Either way, quality of life is reduced - and more than in your scenario, because the features do not benefit from economy of scale as much when they're optional.
> Not making them mandatory means they remain expensive extras, more expensive than if required on all cars.
If they were rare they might be more expensive, but otherwise it's the same system and will cost about the same amount to incorporate into a vehicle. Making it mandatory doesn't make it any cheaper. (Usually the opposite, in fact. From the POV of those manufacturing the safety systems, there's nothing quite like having the government mandate that people buy your product.)
Economy of scale is not guaranteed, or even really all that common once you get past early research and development or one-off custom products and into the realm of mass production. In general commodities become more expensive in response to higher demand, not less. If these systems are something most people actually want and consider worthwhile then they'll already have the benefit of any economy of scale which may apply without the mandate.
Given your car’s safety features have significant externality effects on other citizens, yeah, the government seems well within its duties to mandate certain safety tech.
There is no negative externality merely from not having a safety feature. There would only be a negative externality if there were actually a situation where someone was harmed due to the lack of a safety feature (or any other reason, really) and the person responsible for the harm was not held liable for making the injured party whole.
Mandatory safety features, on the other hand, are a direct example of a negative externality: Those wanting the safety features to be mandatory get what they want while other people bear the cost.
Obviously I can't. The same goes for the case where my car with auto braking kills you—auto braking reduces the risk slightly but doesn't offer absolute protection. Either way the liability for your death is mine as the operator of the vehicle, though if it can be traced to a defect in the auto-braking system I might be able to redirect that liability to the manufacturer. There are any number of other ways you could die due to an accident or negligence, and we can't make you whole for those events either. The best we can do is try to atone for it with respect to those you leave behind—to make up, as much as possible, for the hole your absence leaves in society. That doesn't mean we paralyze ourselves with inaction or always prioritize marginal improvements to safety (even others' safety) over all other concerns.
> That doesn't mean we paralyze ourselves with inaction or always prioritize marginal improvements to safety (even others' safety) over all other concerns.
Frankly I don't care if a (private) road owner wants to require specific safety equipment on the vehicles allowed on their roads. Their property, their rules. Other potential users of the roads can then decide whether they're willing to accept the residual risk. But that argument depends heavily on those who are being excluded (for lack of required safety equipment or low risk tolerance) not being forced to pay for those roads' construction and upkeep. It doesn't work for a government mandating systems to be installed on all vehicles, or for tax-funded roads.
There was massive resistance against seatbelts when they were introduced. Maybe sometimes the government does know better than those actually affected.
On the other hand, making them mandatory makes cars more expensive, which means (especially during a shortage) that some can't afford them at all while the rest are forced to do without other things to compensate; either way it reduces their qualify of life. But of course the government knows better than those actually affected by these laws where the ideal balance lies between safety and QoL.