A Google self-driving car was pulled over by police because the vehicle was traveling too slowly, officials said.

The officer in Mountain View, California, noticed traffic backing up behind the prototype vehicle, which was traveling 24 mph in a 35 mph zone, the force said.

A prototype of Google's self-driving vehicle. ELIJAH NOUVELAGE / Reuters

Realizing it was a Google self-driving car, the cop decided to pull it over "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," the police department said in a blog post.

The Google Self-Driving Car Project responded in a post on its Google+ page, explaining that the speed of the prototype vehicles had been capped at 25 mph for "safety reasons."

"We want them to feel friendly and approachable, rather than zooming scarily through neighborhood streets," it said.

"Like this officer, people sometimes flag us down when they want to know more about our project," it added. "After 1.2 million miles of autonomous driving (that’s the human equivalent of 90 years of driving experience), we’re proud to say we’ve never been ticketed!"

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Google says its driver-less vehicles are currently out on the streets of Mountain View and Austin, Texas. Its fleet includes modified Lexus SUVs and a bubble-shaped prototype, which was the model pulled over by the traffic officer. The company says there are "safety drivers" on board all vehicles — "for now."