A self-driving car that sits idle is a wasted resource, so the owner will always want to put it to some profitable use by moving people or goods. Contrast that to human-owned vehicles that spend 95% their life parked: https://www.reinventingparking.org/2013/02/cars-are-parked-9... That's a tremendous amount of the earths mineral resources that were mined, processed, and transported (at great carbon expense) to sit around doing nothing.