Google Self-Driving Car Pulled Over By Police

US police have pulled over a Google self-driving car because it was going too slowly.

The officer stopped the prototype vehicle in Mountain View, California, on Thursday as it travelled 24mph in a 35mph zone, causing a back-up of traffic.

The police department said in a blog post: "As the officer approached the slow moving car he realised it was a Google Autonomous Vehicle.

"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 Google Self-Driving Car Project responded that the prototype vehicles had a maximum speed of 25mph for "safety reasons".

"We want them to feel friendly and approachable, rather than zooming scarily through neighbourhood streets," it said in a blog post.

Google 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!"

The technology giant's driverless vehicles are currently being tested on the streets of Mountain View and Austin, Texas, it says.

The model pulled over by the traffic officer was one of Google's new prototype vehicles purpose-built to be fully autonomous.

The company says there are "safety drivers" on board all test vehicles.

Google plans to make these cars available to the public by 2020.