[jannyfer]

That would be a bad design for an A/B study (and NYC congestion pricing is not a “study” anyway), because cities are few and not alike and have an enormous list of other things that are different. What NYC equivalent would you pick?

In any case, not every policy change needs to be an academic exercise.

[bunderbunder]

Yup, that is indeed a part of the problem. You'll notice I did say, "Obviously not feasible in practice."

I've got a textbook on field experiments that refers to these kinds of questions as FUQ - acronym for "Fundamentally Unanswerable Questions". You can collect suggestive evidence, but firmly establishing cause and effect is something you've just got to let go of.