Illah Nourbakhsh, a professor of robotics at Carnegie Mellon University and author of the book ‘‘Robot Futures,’’ uses the metaphor of the perfect storm to describe an event so strange that no amount of programming or image-­recognition technology can be expected to understand it. Imagine someone wearing a T-­shirt with a stop sign printed on it, he told me. ‘‘… The very unlikely will happen all the time.’’ The sensory limitations of these vehicles must be accounted for, Nourbakhsh explained, especially in an urban world filled with complex architectural forms, reflective surfaces, unpredictable weather and temporary construction sites. This means that cities may have to be redesigned, or may simply mutate over time, to accommodate a car’s peculiar way of experiencing the built environment.