Don't. Cost isn't stopping people today from ridesharing. Carpooling won't become cool just because there is no driver, in fact I could see it becoming less cool as you will be much more alone with strangers. Many people today would be uncomfortable on a bus at night without a third party present (ie the driver, or at least many other strangers).

Think about all the extra traffic from autodriver cars doing jobs that today are too expensive. And the double-commute possibility. Your car takes you to work, drives itself home to park/recharge, then drives back to pick you up at the end of the day. Any 2-way trip could become 4-way, doubling the per-trip traffic. Or, rather than pay for parking while you shop downtown, you ask your car to drive around in circles until you are ready to go home.