In sizable cities summoning a car, self driving or taxi, is and will be quite quick.
In rural areas and smaller towns, forget it. If you live 20 miles from a big city and want to go shopping, the bus will be faster than summoning a ride. Faster yet will be to continue to own car.
Which is why I said that the *majority* of people will no longer want to own a car - not all.
You are very optimistic about the frequency of busses 20 miles from a city. My mum lives 7 miles from the centre of Worcester where there are 3 busses per day. It's about a 15-20 minute drive.
So that's a win for an autonomous car...