[jaclaz]

But that is a "next" step, unrelated to cars being driverless, sharing fleets (normally human driven) do exist, and they do actually work for some uses, but still they are either on the road or in a parking.

Let's play it another way, let's assume that no self-driving car exists but that all humans (with the exception of those making a living as a driver) can afford a third party human driven car for all their movements/commuting.

This could be either an enormous fleet of Taxis (or Limousines) or Ubers, or each household having one or more cars and a corresponding number of employed drivers.

Still, if the way the city/economy works there is a need that 100,000 people move from point A to point B in a time range between 7 and 9 in the morning and that the same number of people go back from B to A between 17 and 19, the same amount of roads (and the space occupied by them) will be needed.