A lot of discussion and ethical thought about self-driving cars have focused on tragic dilemmas, like hypotheticals in which a car has to decide whether to run over a group of schoolchildren or plunge off a cliff, killing its own occupants. But those sorts of situations are extreme cases. As the most recent crash – in which a self-driving car killed a pedestrian in Tempe, Arizona – demonstrates, the mundane, everyday situations at every pedestrian crossing, turn and intersection present much harder and broader ethical quandaries. As a philosopher working with engineers in Stanford's Centre for Automotive Research, I was initially surprised that we spent our lab meetings discussing what I thought was an easy question: how should a self-driving car approach a pedestrian crossing? My assumption had been that we would think about how a car should decide between the lives of its passengers and the lives of pedestrians.

A lot of discussion and ethical thought about self-driving cars have focused on tragic dilemmas, like hypotheticals in which a car has to decide whether to run over a group of schoolchildren or plunge off a cliff, killing its own occupants. But those sorts of situations are extreme cases. As the most recent crash – in which a self-driving car killed a pedestrianin Tempe, Arizona – demonstrates, the mundane, everyday situations at every crosswalk, turn and intersection present much harder and broader ethical quandaries. As a philosopher working with engineers in Stanford's Center for Automotive Research, I was initially surprised that we spent our lab meetings discussing what I thought was an easy question: How should a self-driving car approach a crosswalk? My assumption had been that we would think about how a car should decide between the lives of its passengers and the lives of pedestrians.