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.

Please explain how you’re going to make me whole if your car without auto breaking kills me.

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.

The best way can do is prevent it with a baseline of good safety tech so that your willingness to save a buck to put others at risk is not allowed.

As I said:

> 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.

It works just fine. See seatbelts.

You’re asserting strongly that the changes are marginal. And sure, the efficacy of the mechanism should be considered. But the point remains.

Your individualism is irrelevant.