They argued against mandating helmets in cars. Your point was that we could do something better than helmets and less inconvenient, namely airbags. But that is an argument for not mandating helmets. And the same argument generalizes to any specific technology. What happens when someone comes up with something better than an airbag but then it doesn't satisfy the rule "is an airbag" and is therefore prohibited?

> Meanwhile, there is a general necessity defense to violating all traffic laws for your motorcycle example.

In practice this kind of general exception either consumes the rule or isn't available when it legitimately applies. If you can say you lost your helmet and would have been stranded then everyone says this and you can't enforce the law. But if you can't claim that when it's true then the necessity defense isn't meaningful.