If the cars aren't parked on the street, then they are going to be driving around constantly. They will still be taking up roadspace, surely?
I don't know what road parking is like where you live, but here in St. Louis they literally take up a lane on the road. The place where they park is marked as a lane and if there are no cars can be used as a lane, but it is also zoned for parking. It seems really stupid, but if those cars aren't parked, it frees up a lane. Even if they're on the roads, they are likely not all on that road at the same time, so the load is spread out.
Plus more car use = wearing out faster = more car production = more environmental costs. I still don't get the argument that self-driving-cars means fewer cars. Why would that be the case? On the contrary, it means car use will increase as it won't be limited to those able or willing to drive themselves, and may indeed involve empty cars travelling to-and-fro to pick people up.
I was making the assumption that we were talking about electric cars, which as I understand them are less prone to mechanical failures. Additionally, they'd be able to ferry themselves to a garage for pickup.
And I don't quite get the point about stop-lights either. Without stop lights, how will any pedestrian cross the road? If the cars are programmed to always stop when any pedestrian is detected, then there will pretty soon be complaints from car users about peds constantly walking in front of them and making it hard for them to get anywhere. Next step will logically be stronger laws to ban pedestrians from the streets or an intentional weakening of the cars' willingness to stop rather than drive into things.
This is a problem I haven't heard a good solution for in cases where there are a lot of pedestrians. At those intersections you'd basically have to have a stop light or some analogue.
The whole thing just looks way more complex and fraught than the hype suggests. Though I admit I am exclusively thinking of crowded urban areas, not so much 'interstates' and rural highways (the non-urban world is a foreign country to me)
I don't think it's going to be nearly as easy as everyone suggests. I keep hearing about cities, but I think driverless cars will take off in the suburbs and "easy" rural areas first. Unfortunately for the US, our infrastructure is built around cars and very few cities have decent mass transit. Even in DC the metro isn't great. I'm not a big fan of large cities, I don't really understand why anyone would want to live or work in one, but if we're going to keep cramming millions of people into small areas, we're going to have to re-think the way we live and commute, which people aren't going to do unless they're forced to.