I agree that the autonomous cars are likely to cause a shift away from car ownership, reducing the total number of cars (which reduces the impact of making all these cars). It might also drastically cut down the required size of parking lots, which especially in America might be a big improvement.
But if you own the car, it's just waiting wherever you left it. If you have an autonomous cab, it has to make an extra trip from wherever it dropped off the last driver to wherever it's picking you up. That alone increases the number of cars on the road. And that's before you consider the cab potentially driving a holding pattern when nobody is actively using or calling it.
But most of all roads are governed by induced demand. People would take a lot more and longer trips if there was the option to just teleport to the destination. The main downwards pressure on the number of trips is the time investment. That's why adding more lanes to roads often doesn't reduce traffic (outside of a short adjustment period): faster trips means more people willing to take it, which fills up that lane. But a trip people weren't willing to do for 40 minutes behind the wheel they might take if it's instead 60 minutes watching Netflix in a driverless car. Which makes the roads fuller and thus slower for everyone.
But if you own the car, it's just waiting wherever you left it. If you have an autonomous cab, it has to make an extra trip from wherever it dropped off the last driver to wherever it's picking you up. That alone increases the number of cars on the road. And that's before you consider the cab potentially driving a holding pattern when nobody is actively using or calling it.
But most of all roads are governed by induced demand. People would take a lot more and longer trips if there was the option to just teleport to the destination. The main downwards pressure on the number of trips is the time investment. That's why adding more lanes to roads often doesn't reduce traffic (outside of a short adjustment period): faster trips means more people willing to take it, which fills up that lane. But a trip people weren't willing to do for 40 minutes behind the wheel they might take if it's instead 60 minutes watching Netflix in a driverless car. Which makes the roads fuller and thus slower for everyone.