Not a foregone conclusion, at all. The average car occupancy now is something like 1.2 people, and self-driving cars might drop that below 1. Time behind the wheel is a cost that people pay for mobility, among other costs, and the Jevons Paradox says that if you make a commodity cost less per unit (i.e. more efficient) we end up using more of it in total, e.g. coal, or lighting. We could have more traffic as people send their empty cars on errands, for example. To get the benefits, you’d have to ban private car ownership. That seems like a heavy political lift, considering that they don’t even expect half of the U.S. private auto fleet to be electric before 2050, and those are available for sale right now.