Also where these miles happen, and how long they take relative to a human driver (1.1x?) matter quite a bit.
edit: I've never shared a road with a self-driving car but I can't imagine I'd like it. I'm extremely patient with fellow meatbags (I work a lot of customer service, and enjoy it), but I'm not at all patient with software thinking it's smarter than it is. I also think people tend to underestimate how much "car-body language" is involved with driving as well, but I suppose that's the primary thing self-driving cars are learning.
Self-driving cars can also burgeon a surveillance state depending on how the fleet is implemented, particularly around what will be required for client-side redaction (pedestrians, license plates, windows etc). Keeping raw data locally for a period of time is fine in case of crash/malfunction, even backing it up encrypted with ephemeral keys can be well and good. But unfettered collection and access to unredacted data—perhaps something Google is drooling over—needs to be stopped cold.