It was seven years ago when Waymo discovered that spring blossoms made its self-driving cars get twitchy on the brakes. So did soap bubbles. And road flares.

New tests, in years of tests, revealed more and more distractions for the driverless cars. Their road skills improved, but matching the competence of human drivers was elusive. The cluttered roads of America, it turned out, were a daunting place for a robot.