I agree with your goals (car free cities), but I don't see this happening. Imagine a neighborhood goes car free - in LA, probably 75+% of people living in that neighborhood work in some other neighborhood. What are they supposed to do to get to work? Public transportation? It doesn't exist, or takes 4x as long as driving.
So we build public transportation up to allow for the massive increase in people using it in that neighborhood? Well, the neighborhoods they're trying to commute to still all use cars, and there's no room for that kind of increase in public transportation there.
So we need some kind of massive public infrastructure project to create the kind of city-wide transportation that's needed to enable this kind of shift? Well, sure. But that takes decades and billions of dollars. And that's if there's the political will to make this happen at all. I live in LA (and commute to another neighborhood to work via public transportation); honestly I don't see it happening.
Greater LA is home to nearly 20 million people. It's entirely feasible to create several car-free neighborhoods, each with more people than Omaha Nebraska. That's plenty enough people for transportation solutions to evolve organically.
Something to keep in mind is the existing suburban infrastructure is also unsustainably expensive. It's quite likely that many American suburbs will decay in the coming decades, a la Detroit, as poor residence can't supply enough tax dollars for maintenance.