The best long term solution for both nicer cities, happier people, and less environmental damage is to overhaul our infrastructure. Don't build trains in car dependant cities, make the car dependant cities walkable with public transportation that will leave you within a few minutes of your destination. The real reason self driving cars are the "future" is because selling cars has a higher profit margin than train/bus tickets.

There's an argument to be made that driverless cars make more efficient use of our existing infrastructure, namely, roads

Buses. It's almost driverless car with 1/80th of driver per ~~driver~~ passanger. Also it's 1/80 of car per car equivalent.

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.