A self-driving vehicle made by Uber has struck and killed a pedestrian. It’s the first such incident and will certainly be scrutinized like no other autonomous vehicle interaction in the past. But on the face of it it’s hard to understand how, short of a total system failure, this could happen when the entire car has essentially been designed around preventing exactly this situation from occurring. Something unexpectedly entering the vehicle’s path is pretty much the first emergency event that autonomous car engineers look at.