Another commenter shared Tokyo, and I can explain what they did:

1. Tokyo as a whole made it extremely difficult to own cars. To own one, you need to prove to the government that you have a dedicated parking space. There’s also a high tax on it and on insurance premiums. My Japanese friend told me how his family used to car share with the entire neighborhood. Uncertain if this is a Tokyo thing.

2. Public transportation is ridiculous. You are always about a 10 minute walk to a subway or bus.

3. Highways are underground, which encourages cars to get out of the street level.

By not having 15 million citizens in one city?