I don't expect to see autonomous vehicles as private cars in cities, except possibly at the very high end. More likely, they will be in fleets and offer on-demand transportation, like Uber or even taxis but with no driver. No need to park them at all if they go to their next pickup. I think this will be basic to the economics of their success--it will reduce the cost of transport while benefiting the environment. It might mean more cars on the road at once in dense urban areas, but there would be less need for parking spaces and inter-car communication could reduce congestion and improve traffic flow.