One morning in June, a self-driving car prototype developed by Alphabet’s Waymo unit drove itself northbound on the freeway not far from the company’s headquarters in Mountain View, Calif. The Waymo vehicle, a white Chrysler Pacifica minivan, traveled on a route that the company uses to test different versions of autonomous vehicle software.

Behind the wheel of the Pacifica was a human driver who, after about an hour on the road, appeared to doze off during the drive, according to a person who has seen a video of the incident and another person who was briefed about it. In doing so, the driver inadvertently turned off the self-driving car software by touching the gas pedal, and then failed to take over the wheel, another person familiar with the matter said. The van crashed into the highway median.