Cleaning comes to mind as a serious, unsolved problem here. The reason I like my own car is because I know the history of its dirt.

I wonder what the worst case scenario is here. I'll put Muni-level syringes and human waste as the worst. But (1) fare evasion wouldn't be a problem and (2) the 1% of worst offenders who account for 99% of the disgustingness can be readily blacklisted.

I suspect the typical case will be a drunk person throwing up in it on the way home. Hopefully they self-report; if they don't, it comes to the next rider to report it, who does and gets a generous reimbursement, charged to the credit card of the last rider.

But who's going to want to deal with that? Not most people. I would not rent out my car for the same reasons I'm not a landlord.

Another reason why private ownership won't make sense. You don't clean it, X Company does.

I've been using "car-sharing" (really an unfortunate mis-nomer) systems for some time now. Cleanliness has never really been an issue.