Also, one thing that's often forgotten is that roads take up space. A lot of it. You make your roads bigger to accommodate more people, and all of your buildings wind up farther apart as a result. When buildings are farther apart you have to drive farther, meaning that everyone's journeys are longer, meaning more traffic... and on and on it goes.
I mean, at what point do you just build a 400 square mile skid pad with nothing else there just to "alleviate traffic"? Hell, that's practically what Orange County is already.
Assuming "parking spaces" include one's home space (like garage or reserved spot), 3 should be expected, at least: home, work, and wherever you're visiting.
Now assume Uber et al own fleets of self-driving 9-passenger minivans. During peak commuting hours they're completely full because they pick up different passengers who have the same commute, and that way you eliminate the parking space both at home and at work.
The rest of the day they don't actually park anywhere, they just stay on the road operating by carry one or two passengers at a time instead of eight or nine. Or half them stay on the road operating and the other half go off and park in some huge lot out where land is cheaper until demand picks up again.
Then instead of 3.3 spaces per car you can have <1, and most of them can be in low land cost areas.
It's actually kind of like dynamically allocated mass transit.
More likely scenario will be self driving cars cooperating with each other to drive from start to finish without any stops. Whether on freeways or local streets, cooperation amongst vehicles will raise the average speed and the volume of vehicles you can process through a given area. Pools work to a certain degree if everyone is starting and ending at the same location. When they don't, then it actually takes longer than driving by yourself.
Final point, you can certainly build out less parking spaces, but pre-existing spaces won't go away without redevelopment.
14 percent of LA county (not just city!!) is parking. http://www.citylab.com/commute/2015/12/parking-los-angeles-m...
I'm trying to find a better source, but at one point supposedly 59 percent of the central business district was car infrastructure (parking, roads, etc.) http://www.autolife.umd.umich.edu/Environment/E_Casestudy/E_...
I mean, at what point do you just build a 400 square mile skid pad with nothing else there just to "alleviate traffic"? Hell, that's practically what Orange County is already.