People who advocate this line of thought imagine a density of cars so high that if a car is dirty, you reject it and a replacement is 2 minutes away; and if you need a child seat and a bike rack, a car with both fitted is only 2 minutes away, and so on. They imagine a street like [1] but with every yellow cab an instant-hire robot car.

Obviously, it's difficult to square that theory with the "Public transit won't work in America as the population density is too low" line of thought so you probably have to choose one or the other.

[1] https://www.alamy.com/stock-photo-busy-street-scene-in-city-...