autophil probably goes too far, but he isn't entirely wrong--even perfectly efficient robotic cars powered by pixie dust are inefficient and not a panacea for the problems of traffic and pollution that car-centric infrastructure has caused.
Today car companies sell cars based on many criteria. Fuel efficiency is one of them but when gas prices aren't too high, individual customers don't care about that very much.
Someone who operates a fleet of 10.000 cars will make fuel efficiency their top-most priority because that will drive down the cost of their service and the cost will be a major reason why customers will pick one provider over another, not how stylish the car is or how fast it goes from 0 to 60.
E.g. if they realize that by putting solar panel on the roof of the car they can save money in the long term, they'll immediately put solar panel in every car. The progress in making cars more efficient will be much faster.
This isn't to say self-driving cars are useless. They might be just the kick needed to get people out of car ownership, which will enable the fundamental land use and infrastructure changes necessary to make more sustainable improvements. But they aren't the endgame for sustainability.