I have a better idea that we can implement right now without waiting for autonomous driving to become a real technological capability.
We could make big cars with many seats and have them driven around town by humans- just a single human could drive a single vehicle capable of transporting 50, maybe 100 others. We could have those big cars stopping at predetermined points around the city, so that people would know where to get on and off. And we could support that service by asking the users to pay a small fee upon boarding the big car. This would cover the costs of the driver, and the big car, and still leave some to pay for infrastructure, etc.
These big cars would probably have relatively restricted areas of operation, but, for the foreseeable future, so will autonomous cars (which have to be geofenced) and, like I say, we already have the technology for those big-cars-with-many-people-sitting-in-them-and-a-single-driver that I'm talking about.
And since a whole bunch of people would be carried by one or two of those big cars per day, instead of each riding a single (autonomous or not) car, we could drastically reduce the number of automobiles-per-person, and, therefore, the CO2 emissions, thereby reducing pollution and protecting the environment to boot.
So? Anyone want to invest on that?