Police in the Northern California city of Mountain View saw something unusual on the road on 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 on Thursday when police officers stopped one of the cars because it was going 24 miles per hour in a 35 m.p.h. 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 m.p.h. or slower, the police said.