The most relevant stat though is the number of miles of car usage. If there are ~20% the number of cars, but each car is used 10x as much, we're worse off. If nobody owns a car but always calls a cab, the cab might do twice as many miles deadheading to the pickup. And instead of lasting 15 years, a typical car might only last 2 years because it's getting 10x the milage per day. So fewer cars might result in more gridlock, noise and tire particulate pollution. Fewer cars might mean just as many cars built per year.

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.

Where I live, I can get a cheap taxi, any time, to anywhere I might want to go, through an app. The only difference between that and Waymo seems to be that it is controlled by a meat sack rather than a computer. I don't see autonomous cars as all that different to what I have now.

That's less cars in garages, not on the roads.

This means there will be more cars on the road and more traffic. Whether or not people individually own the cars is irrelevant.

If autonomous cars drive down the costs of taking a taxi it’ll mean more people will do that versus public transit.

Anything that reduces public transit use or increases individual car use will be disasterous for traffic and transportation in our cities.

But just because fewer people own one personally, will that necessarily mean fewer cars on the road? Might still be an increase in cars, but with a different ownership model. It's tough to make predictions, especially about the future.