As another commenter opined, in retrospect the greatest failure seems to have been missed planning to evolve suburbs into cities of their own. An alternate rea

As another commenter opined, in retrospect the greatest failure seems to have been missed planning to evolve suburbs into cities of their own. An alternate reality would have seen the US purchase rights of way between traditional urban cores and new suburb centers, then run rail (and upzone density around that rail) once that suburb attained sufficient population to merit. Then build sufficient park-and-ride capacity to handle the suburb's population's last mile needs. So basically, what China does. Except with less upfront heavy construction (because the US doesn't need to drive a demand-based economy) and more emphasis on right of way purchase (because the US has stronger property rights). Instead, we largely grew the first generation of suburbs and then... just didn't invest or plan non-road transit. And so began growing suburbs outside of suburbs, and none of them connected with a high-throughput transit network.