If the taxi is soiled then the next passenger indicates this via the app. The taxi is sent for cleaning and the previous passenger is charged for the cleaning. Also, there will be cameras inside in case it has to be proven who made the car dirty.

If people have to pay the cleaning bill they will be careful not to make a mess.

It'll probably just be smooth, hard plastic.

Well, considering that these won't allow anonymous usage this can be mitigated by kicking out "bad" users and big fines. Also cars can visit the cleaning zone frequently.

For "free floating car sharing" (DriveNow/ShareNow, Yandex.Drive, ...) this seems to work quite well.

This isn't much a problem in terms of self-driving, but solutions to this aren't hard; eg. someone could use the app before they enter a self-driving taxi to report that the car has stains, trash, food, etc. left in it. The company behind it could then queue it to be cleaned.

> I’m imagining how gross a taxi would be that doesn’t have a driver to provide passengers a low level of behaviour moderation.

I've talked to a number of uber drivers. The cameras recording the passengers are a much greater inhibitor of bad behavior than the driver.

Except if you are living in Japan.