Carmakers have always designed vehicles for individual drivers and families, not groups of strangers. That could pose a problem going forward. In fact, it already has. Look at the tensions that arose when services like Uber transformed personal rides into public transport. (Lack of privacy, for starters, driver mix-ups for another.) The advent of self-driving cars—vehicles for which codified user interfaces and social norms have yet to emerge—will only complicate matters further. As ride-sharing becomes more popular and autonomous vehicles more capable, one thing becomes clear: Society needs a new kind of car.