Police in the Northern California city of Mountain View saw something unusual on the road Thursday: A car was moving too slowly, causing a traffic backlog.
So they pulled over the vehicle and peered inside.
It was a self-driving car.
Google's autonomous vehicle project, which has logged 1.2 million miles, was nearly handed its first traffic ticket Thursday when police officers stopped one of the cars because it was going 24 mph in a 35 mph zone.
"The officer stopped the car and made contact with the operators to learn more about how the car was choosing speeds along certain roadways and to educate the operators about impeding traffic," a police statement said.
Under the California Vehicle Code, the cars are permitted to operate on streets that have a speed limit of 35 mph or slower, police said.
So no ticket was issued. The car technically had no driver, but there are usually two operators in the Google cars capable of taking over if needed, and that was the case this time, the police official in charge of the traffic team, Sgt. Saul Jaeger, said in a telephone interview.
The Google vehicle was allowed to go on its way, with the understanding that if the operators noticed that traffic was stacking up, they needed to pull over and let it flow by, just as if someone had engine trouble and was inching down the road.
"Just like anybody," Jaeger said.
Google's autonomous test cars are programmed to follow the letter of the law. But as The New York Times reported in September, researchers in the fledgling field of autonomous vehicles say that one of the biggest challenges facing automated cars is blending them into a world in which human drivers don't behave by the book.
Since 2009, Google cars have been in 16 crashes, mostly fender-benders, and in every single case, the company says, a human was at fault.
Google, responding to Thursday's episode, said its vehicles had never been issued a ticket.
"Driving too slowly? Bet humans don't get pulled over for that too often," a Google blog post about the episode said.
"We've capped the speed of our prototype vehicles at 25 mph for safety reasons. We want them to feel friendly and approachable, rather than zooming scarily through neighborhood streets. Like this officer, people sometimes flag us down when they want to know more about our project."
Zandr Milewski photographed the car that was stopped in Mountain View, which happens to be the home of the headquarters of Google's parent, Alphabet, from a nearby office building, The San Jose Mercury News reported. He was working on a project in a conference room when a colleague told him what was happening outside.
"We all immediately dropped what we were doing to go look," Milewski told The Mercury News. "It's not something you see every day."