I think you're right that autonomous cars will lead to more vehicle miles traveled, because people will be sending their vehicle off on their own, but I think it's ambiguous as to what that will mean given the efficiency gains involved.

That said, I have a hard time imagining them leads to owning more cars. First, there's the hole taxi-like thing, which might make it easier not to actually own one (while I agree, this will not be anywhere near universal). Then there's the fact that your car won't be stuck at the office with you, so it can go get grandma and Junior. Heck, even married couples might be able to stagger their commutes so they can both get by with one car. So, yeah, my guess is yet fewer cars per person, not more.

As to longer commutes, it really depends on whether we decide to continue to subsidize them.