After a closely watched vote, driverless cars, once a Silicon Valley fantasy, have become a 24-hour-a-day reality in San Francisco. Are autonomous vehicles an interesting and safe transportation alternative? Or are they a nuisance and a traffic-blocking disaster waiting to happen? Cade Metz, who covers technology for The Times, describes the unique challenges of coexisting with cars that drive themselves. Guest: Cade Metz, a technology correspondent for The New York Times.