Re: Own the box rent the motor (again)
People can own a standardised passenger compartment* and call up the autonomous motive unit as required and to suited to the trip.
Wouldn't that be the worse aspects of both systems.
+ No Parking Space Is Saved
1) Because you still have to store the passenger compartment at home, office and shopping center.
2) Still need multiple passenger compartments for multiple simutaneous trips (office and school run)
+ Up Front Costs
1) You still have to pay a large upfront capital payment to buy each passenger compartment your family would need. They would be cheaper than a whole car but not as much as you might think. They would need to be crash impact resistant compartments in the same way as cars are now so they will still be significant lumps of metal.
+ Still Dependant On Someone Elses Schedule
1) So you've got the cost of buying, storing and maintaining your own passenger compartment, but you are still dependant on someone elses schedule before you can make a trip, if there are enough motor units to nearby that wont be a huge problem, but image you live in the sticks and need to get into town, the nearest motor unit might be the town you want to visit, so you order a unit, it travels from town to your house, back to town, goes off to do other trips, then picks up your passenger compartment again to travel from town, to your house, and back to town again (a single 2-way trip needing a minimum of 6 journeys)
A final proof that this idea wouldn't work is this. Nothing in your suggestion couldn't be done now using small tractors units with drivers instead of autonomous cars and yet no one does it... anywhere in the world as far as I can see. The closest analogy is 40ft goods containers but they work because they are built in places with lots of space to store containers, lots of expensive machinery can be concentrated into one place to store and stack the containers and transport time isn't as critical as it for passenger journeys.