The transition to self-driving cars, when and if it happens, will represent what may be the single greatest change to the world of vehicle design. Car designers are thrilled about the opportunity, as they typically are about any so-called “white spaces”—flying cars, invisible cars, three-row crossover landaulets. But they are also daunted by such a future, in part because, as with any radical transition, there are a multitude of variables, not just in terms of capability and technology, but also consumer preference and behavior. And many of these core issues are as-yet entirely unclear. “I look at autonomous as being a bigger change in human interaction with mobility than when we went from a horse and carriage to a horseless carriage,” says Stuart Norris, General Motors’ director of design for advanced mobility and experience. “And of course, we don’t know what we don’t know. Nobody is running autonomous vehicles at scale. You can’t just go to a traditional market-research event. The market doesn’t exist.” Obvious and elemental among both the opportunities and the challenges is a wholesale rethinking of the notion...