> Guess what, people would absolutely get sent to the major roadways if (and only if!) those roadways would actually get people where they want to go, faster. If those roadways suck compared to the alternatives, then that points towards a duty of the major to improve those major roadways

I strenuously disagree. You seem to be saying that city planners and traffic engineers have to design with the constraint that a large subset of drivers will take the instantaneously fastest route rather than sticking to intended high-throughout roads.

I suspect that this would have rather poor implications. It means that little residential streets that happen to parallel major streets will get used at their full capacity whenever the major streets drop below 25-ish mph.

This, in turn, likely means that the little streets can’t safely be used by pedestrians and become unsafe for people trying to cross or exit driveways, let alone for kids who want to play.

This sucks. It’s not clear what cities can do about it, but blaming the cities and saying that every last through street needs to be widened is not the answer.