If entity that provides those cars is big enough, they can match up the destination and optimize the traffic so that people going to the (roughly) same place share a car. When I was commuting from SF to MV for work, I had to use the whole car. There were plenty of other people who were commuting at the same time from/to very similar destination and used the whole car. It's not hard to algorithmically put several such people into the same car.
Today cars are 4/5 seaters so you get at 3x reduction compared to current levels (taking into account that not everyone drives by himself today) but you could easily redesign the cars to not be much larger and taking 8-10 people. Or make them straight up buses. Google already does that with their shuttles where they pick e.g. people from SF and drive them in big buses to MV, except it would be much more efficient because the potential pool of people transfered would not be just "people who work at Google and live in SF" but "every person who lives in SF and works somewhere in MV", which is a much bigger number.