In Manhattan it's 23% (http://www.nycedc.com/blog-entry/new-yorkers-and-cars). This is quite scarce relative to the rest of the USA and indeed even relative to the suburban boroughs of the greater New York City metro area. For the US the last average I saw (World Bank data http://web.archive.org/web/20140209114811/http://data.worldb...) was 81%.

I should have said Manhattan. 23% is "scarce" in that, on the island, there are relatively few gas stations, parking is expensive, and getting your car serviced reasonably encumbering. It's also a four-fold reduction from the U.S. average.

How much cheaper would a driverless Lyft or taxi be? 50%? How many more people would forego driving, in Manhattan, if ridesharing were half as expensive as it is today? If half do we're at 11.5% car ownership, a 7-fold reduction from the present-day U.S. average. If 2/3 do, we get to 10-fold.

Yes, I agree with your premise here. Personal anecdote, I lived in San Francisco in the city 8 years. It was by far more economical for me to have both Zipcar and taxis (later UberX and Lyft) combined per month than to have a vehicle lease + parking costs + insurance/maintenance/gas. By far. In both $ and time.

I'm not certain that other elements of your vision work in non-metro areas.

What percentage of people live in metro areas though?

In 2010, 80.7% of Americans live in an areas with an overall population density of 2,534 people per square mile [1].

[1] http://www.reuters.com/article/usa-cities-population-idUSL2E...

It's probably also worth noting that a lot of the time when people say "NYC," they really mean Manhattan--and, more specifically, they're really thinking Manhattan below 110th Street or so. I imagine the number is significantly lower if you remove the other boroughs.

Not even all of the other boroughs, either. Eastern Queens and Staten Island basically require a car, and so those two counties have much higher rates of vehicle ownership than the rest of the city. Brooklyn, Western Queens, and the Bronx are much, much more friendly to people who don't want to drive (like me)

According to Wikipedia (https://en.wikipedia.org/wiki/List_of_U.S._cities_with_most_...), the jump from NYC (56% car free) to the next most car-free city (38%, Washington DC) is pretty massive.

That doesn't make car ownership scarce. It makes it low.

...because in most of the country each household tends to own more than one care? Are you kidding me?

Relative to other areas, it seems scarce to me.

When something is scarce, this implies there is scarcity, or inability to meet demand. Colloquially, "scarce" is used synonymously with "rare", but rare isn't a good word choice for something occurring 50% of the time.