It's pretty easy to do, other than the politics, in the highest growth cities. They tend to have so many areas that are severely underdeveloped that it would be economical to upzone them with transit in mind. Many a European city is far less car-centric than they were 30 years ago, just via rebuilding streets that were already dense enough to make sense. A good 50% of my home town is already just for pedestrians and delivery vehicles, and I read streets are changing.
The real problem is both cities that are not growing at all, and cities that are still digging into the car hole.
You might be able to rezone 50 year old suburbs, if just because the houses themselves keep losing value (as the real price increases are just land). But when the house was just built 10 years ago, it's a very tough sell. And if you expect a couple of million people, who are living in very low density suburb, to come downtown for any reason, you either make their transit story hell, or you are stuck wasting a lot of space just to manage their cars: Bad either way.
The cities that aren't growing just are going to have a lot of trouble becoming denser, and the political problem will be even bigger, as every effort to make the area near the city center denser in a city that doesn't grow is just making the outer suburbs less valuable. I am really worried about those cities. As having the infrastructure to support four times the number of people that you have is just a road to fiscal ruin.
The real problem is both cities that are not growing at all, and cities that are still digging into the car hole.
You might be able to rezone 50 year old suburbs, if just because the houses themselves keep losing value (as the real price increases are just land). But when the house was just built 10 years ago, it's a very tough sell. And if you expect a couple of million people, who are living in very low density suburb, to come downtown for any reason, you either make their transit story hell, or you are stuck wasting a lot of space just to manage their cars: Bad either way.
The cities that aren't growing just are going to have a lot of trouble becoming denser, and the political problem will be even bigger, as every effort to make the area near the city center denser in a city that doesn't grow is just making the outer suburbs less valuable. I am really worried about those cities. As having the infrastructure to support four times the number of people that you have is just a road to fiscal ruin.