Self-driving, autonomous vehicles — a topic of speculation, if not science fiction, just five years ago — are suddenly going mainstream. Every major car manufacturer is offering prototypes or early models of cars that do most of the driving automatically, including luxury models from Tesla with a built-in “autopilot” feature. Startups and major tech companies, notably Alphabet’s Google X division, are investing heavily in smart car technology, as are network ride-sharing companies such as Uber and Lyft.