TOKYO—A semiautonomous Honda SUV was traveling down a test track at 20 miles an hour in March last year when a child-size test dummy moved into the middle of the road. Oblivious, the Honda mowed it down.

It was part of a brutal day of Japanese government testing for Honda Motor Co., whose vehicle was equipped with a camera and sensors that were supposed to detect obstacles and apply brakes to avoid a collision. The SUV scored 0.2 out of a possible 25 points in the pedestrian portion of the test, the worst among tested vehicles.