They have been on our town streets (Los Altos) for a several years. They left the main freeways and expressways a long time ago. They are very heavily present on our surface streets now.
The one parked in front of our house a couple nights ago did not appear to have a person behind the wheel. Just in front passenger seat and rear seat. But it was dark and hard to see precisely.
And if you had put a cardboard cutout of the rear end of a car in front of it, and a cutout of the front end of a car in back of it, the driverless car would have sat there forever.