Re: No need to worry
I'm assuming an autonomous car will be significantly cheaper to hire than a taxi because the autonomous car doesn't need to cover the cost of a human taxi-driver.
That is an assumption that I have seen used by a number of sources, and may perhaps be a reasonable one to make for a commuter vehicle, but I am not so sure it will be the case and, even if it is, will really drive down private ownership of vehicles. First, many areas that have significant numbers of taxi drivers are sensitive to job loss and may take a protectionist stance on the use of driverless cars as for-rent vehicles, especially in cases of individuals leasing them out during the day. Look for example at where Uber has had legal challenges.
Second, the time when someone is most likely to need a car for rent is going to be during peek driving hours which includes the owner who used it to drive in to work. There will be a smaller demand for these vehicles as rentals than there will be supply, which does tend to drive down prices, but...
Third, while the cost of renting one of these has a good chance of being cheaper than a regular cab, assuming there are no price controls, they will still be more expensive than ownership by necessity. No-one is going to rent at or below cost especially as there is going to be some serious liability issues - who is going to want to risk their personal ride home on making a quick $10 for an unsupervised and potentially un-trackable transaction - will you really want to know what sticky mess on the back seat is when you get your car back in the evening?
I did a quick lookup of the number of cabs in different areas. There are now over 20,000 licensed vehicles on London's roads while New York has over 13,237 cabs and more than 40,000 other for-hire vehicles. On the other hand, there are 2.6 million privately owned cars registered to London residents while around 48% of New Yorkers own cars which puts the number of cars there at over 4 million. Uber has more than 14,000 cars operating in New York and 7,000 in London (Uber claims this will grow to 42,000 by next year), so there is clearly demand, but I would guess that there will be strong pushback in areas where there are established taxi services, precisely where there will be a confluence of available cars for hire and people who might hire them.
I am sure some folks will try this, but that it will turn out to be more hassle than it is worth. My guess is that if you want to make money off self-driving vehicles, then your best bet is to sell the security devices that people will want to install in their cars before handing them over to a stranger site unseen.