AFP/San Francisco

Google playfully posted a photo of a motorcycle cop pulling over one of its self-driving cars on Thursday for riding along at a cautious pace.

"Driving too slowly?" the self-driving car team asked rhetorically in a message that accompanied a picture on its Google+ social network page.

"Bet humans don't get pulled over for that too often."

Police in Google's home town of Mountain View in Silicon Valley explained in an online post that an officer noticed traffic backing up behind a self-driving car going 24 mph on a street with a 35 mph speed limit.

"As the officer approached the slow moving car he realised it was a Google Autonomous Vehicle," the police department said.

"In this case, it was lawful for the car to be travelling on the street."

California law allows self-driving cars to operated on roads with speed limits of 35 mph or slower.

The officer did chat with the occupant of the car about impeding traffic, according to police.

Google caps the speed of its self-driving cars at 25 mph for safety reasons.

"We want them to feel friendly and approachable, rather than zooming scarily through neighbourhood streets," Google said of the speed cap on it driverless cars.

"After 1.2mn 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!"