Maybe Google was right: its self-driving cars do drive too cautiously.

A Google self-driving car was pulled over Thursday for driving too slowly. A Mountain View traffic officer stopped the self-driving car for driving 24 mph in a 35 mph zone, causing traffic to back up near Google’s main campus in California.

Impeding traffic is an offense in California under 22400(a) of the California Vehicle Code. Here’s what the rule says: “No person shall drive upon a highway at such a slow speed as to impede or block the normal and reasonable movement of traffic, unless the reduced speed is necessary for safe operation, because of a grade, or in compliance with law.”

Google’s self-driving cars operate under the Neighborhood Electric Vehicle Definition per 385.5 of the California Vehicle Code and can only be operated on roadways with speed limits at or under 35 mph. However, Google has capped the speed of its self-driving cars at 25 mph for safety reasons.

“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,” the Mountain View police department says.

Google responded with the following note on its Self-Driving Car Project Google+ page:

“Driving too slowly? Bet humans don’t get pulled over for that too often.

“We’ve capped the speed of our prototype vehicles at 25mph 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. 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!?”

Google’s self-driving cars have been involved in 14 accidents, none of which were Google’s fault. Eleven of those accidents have been rear-end collisions where the Google self-driving car was hit by a human driver in another car.