45% of households nominally in New York City have a car and that is by far the lowest of any American city. In the ideal American city for not owning a car, almost half of the households own one anyway! This is about the same in Europe too. Pick a major European city and you'll probably find that between a third to a half of the households in that city own a vehicle, despite the famous European public transit and habit for walking and cycling that online anticar activists love to brag about.

In every other American city besides NYC, most households have a vehicle. And besides virtually every American city save one being built for cars, you also have to keep in mind that the "most people live in urban areas" statistic counts all the people living in the suburbs around cities, e.g. people who have chosen a lifestyle literally built around car ownership.

I don't think that autonomous buses would be as cost-effective as private vehicles, since you'd want someone to be on board for security on a shared vehicle even if they're not in the driver's seat. Labor is the most expensive component of public transit operating costs, at least in my area. That said, in low-density areas it's not uncommon to see just one or two people on a big bus that comes once every 40 minutes in the middle of a weekday, and I'm sure running autonomous taxis instead would provide better service at a lower cost, given the same subsidies.