I believe what they're saying refers to the empty 3 seats you might find in a singly-occupied 4 passenger car, as is often seen commuting. And I basically agre

I believe what they're saying refers to the empty 3 seats you might find in a singly-occupied 4 passenger car, as is often seen commuting. And I basically agree. There is still space in many passenger vehicles. And the vacuum is not strong to get those filled, because it is not infinite or even powerful enough, but there are people trying to fill those spaces to some extent. That was some of the original pitch of the "sharing economy..." That if you had "extra space in your car or house," you could fill it with a rider or a roomer. It's just that managing the complexities of the extra space in a vehicle is very difficult, particularly in comparison to the simplicity of filling any empty Manhattan streets with new instances of vehicles.