In fact, that reinforces my argument: it's not about relative risk, it's about the definition of what's considered negligent varying over time based on changing attitudes.

Zero tolerance laws have nothing to do with impairment. 0.001 BAC doesn’t have a detectable influence on behavior but 7 states set have zero tolerance at literally 0.000. The definition of negligence isn’t changed by these groups, they are going to arbitrarily increase penalties over time because that’s why they exist.

Nobody advocates for drunk drivers as a group so there’s no equilibrium reached, just ever harsher penalties and lower limits. I don’t drink, but it’s obvious that using literally 0 as your threshold will pointlessly waste money going after edge cases unrelated to drinking.