We've self-driven more than 1.5 million miles and are currently out on the streets of Mountain View, CA, Austin, TX, Kirkland, WA and Metro Phoenix, AZ.
It sounds like they have actual cars, driving actual miles, in actual cities. I've also had a coworker who was driving in the Bay Area see one of their cars go by him on the highway - with no one driving.
Now, they might not replace all cars, but even eliminating regular cars in major cities will dramatically change things. Imagine the Bay Area, Los Angeles, Chicago and DC with less than half the cars they have now (due to people sharing, etc.). Suddenly, rush hour isn't a nightmare, and parking spots don't need to sell for $10k/year since it would be cheaper to send your car home instead of parking it - and then it is available for your spouse/child/family member to use, instead of it being parking in a parking ramp downtown. Plus, you could send your car to drive your child to before- and after-school activities instead of doing it yourself.
"Christine, go pick up Carrie from school and drive her to swim practice and then park and wait for her to finish. Then drive her home without stopping at Dairy Queen. (Yes, I named the car Christine.)