Because there's no good way to answer this ethical dilemma. If there is no other choice, will a driverless car crash into a group of children running across the street or divert and potentially harm the driver? The answer, save the life of the passenger, according to Mercedes-Benz's manager of driver assistance systems and active safety. Editor's note: After this article went life, Daimler, Mercedes-Benz's parent company, had several things they'd like to add. First, it says the company is focused "on completely avoiding dilemma situations" and has never made an official decision on the topic.

Suppose that a driverless car is headed toward five pedestrians. It can stay on course and kill them or swerve into a concrete wall, killing its passenger. On page 1573 of this issue, Bonnefon et al. (1) explore this social dilemma in a series of clever survey experiments. They show that people generally approve of cars programmed to minimize the total amount of harm, even at the expense of their passengers, but are not enthusiastic about riding in such "utilitarian" cars--that is, autonomous vehicles that are, in certain emergency situations, programmed to sacrifice their passengers for the greater good. Such dilemmas may arise infrequently, but once millions of autonomous vehicles are on the road, the improbable becomes probable, perhaps even inevitable.