The ultimate car service would allow me to find a car a short walk from where I am and then do a one way trip, without a booking. I've always felt that the only barrier to providing this service was a critical mass of drivers and zipcars. Most cars are currently idle for the majority of their lifetime, taking up a lot of parking space. Imagine if the streets were lined with shared cars instead of privately owned cars. Once you get to a certain density you should be able guarantee availability in all but the most remote locations. There are large scale migrations like commuting that take place over the day/week but the availability of cars should match this. Given a rising population density it seems inevitable that such a service will have to exist as we will run out of storage space for idle cars.