I frequently see people claim that one effect of robocars is that because we'll share the cars (when they work as taxis) and most cars stay idle 95% of the time, that a lot fewer cars will be made -- which is good news for everybody but the car industry. I did some analysis of why that's not necessarily true and recent analysis shows the problem to be even more complex than I first laid out.
To summarize, in a world of robotic taxis, just like today's taxis, they don't wear out by the year any more, they wear out by the mile (or km.) Taxis in New York last about 5 years and about 250,000 miles, for example. Once cars wear out by the mile, the number of cars you need to build per year is equal to:
Total Vehicle Miles per year
Avg Car Lifetime in Miles
As you can see, the simple equation does not involve how many people share the vehicle at all! As long as the car is used enough that the car isn't junked before it wears out from miles, nothing changes. It's never that simple, however, and some new factors come into play. The actual model is very complex with a lot of parameters -- we don't know enough to make a good prediction.
People travel more in cars.
It's likely that the number of miles people want to travel goes up for a variety of reasons. Robocars make car travel much more pleasant and convenient. Some people might decide to live further from work now that they can work, read, socialize or even sleep on the commute. They might make all sorts of trips more often. Outside of rush hour, they might also be more likely to switch from other modes, such as public transit, and even flying. Consider two places about a 5 hour drive apart -- today flying is going to take just under 3 hours due to all the hassles we've added to flying, even with the improvements robocars make to those hassles. Many might prefer an uninterrupted car ride where they can work, watch videos or sleep.
Vehicles run empty to reposition
Regular taxis have wasted miles between rides. Indeed, a New York taxi has no passenger 38% of the time. Fortunately, robocars will be a lot more efficient than that, since they don't need to cruise around looking for rides. Research suggests a more modest 10% "empty mile" cost, but this will vary from situation to situation. If you need the robotaxi fleet to constantly run empty in the reverse commute direction, it could get worse. Among those who believe robocars will be more personally owned than used as taxis, we often see a story painted of how a household has a car that takes one person to work, and returns home empty to take the 2nd person, and then returns again to take others on daytime errands. This is possible, but pretty inefficient. I think it's far more likely that in the long term, such families will just use other taxi services rather than have their car return home to serve another family member.
Cars last longer
The bottom part of the equation is likely to increase, which reduces the number of cars made. Today, cars are engineered for their expected life-cycle -- 19 years and 190,000 miles in California, for example. Once you know your car is going to have a high duty cycle, you change how you engineer it. In particular, you combine engineering of parts for your new desired life cycle with specific replacement schedules for things that will wear out sooner. You want to avoid junking a car with lots of life in the engine just because the seats are worn out, so you make it easy to replace the seats, and you have the car bring itself to a service center where that's fast and easy.