Over the weekend, Internet giant Google revealed it has been secretly developing and testing autonomous robot cars, logging over 140,000 miles so far.

It's a topic rarely discussed in the media, but self-driving cars are probably closer to market than most people realize. Over the weekend, Internet giant Google proved that with the revelation that it has been secretly developing and testing a fleet of robot cars.

The company confirmed its fleet of seven prototypes has logged over 140,000 miles on California roads. 1,000 of those miles were travelled fully automatically, while the remainder was navigated with only partial driver input.

"Our automated cars, manned by trained operators, just drove from our Mountain View campus to our Santa Monica office and on to Hollywood Boulevard," wrote engineer Sebastian Thrun in an official blog posting. "They've driven down Lombard Street, crossed the Golden Gate bridge, navigated the Pacific Coast Highway, and even made it all the way around Lake Tahoe."

Each prototype is equipped with cameras, lasers, radar and GPS to understand its surroundings and navigate safely. The car's sensing capabilities are combined with data stored on Google's servers, which provides advanced knowledge about the road. This includes lane counts and configurations, information on signs, and the location of things like traffic signals.

Google says it recruited engineers from various teams that competed in the DARPA Challenges, a series of robot car races hosted by the U.S. Department of Defense. It's unclear what prompted Google to enter the field of car automation, but CEO Eric Schmidt recently stated "your car should drive itself ... it's a bug that cars were invented before computers."

Thrun indicated Google approached local law enforcement to make police aware of the extensive testing in Los Angeles and San Francisco. The company says there are no laws that prevent self-driving cars as long as there's an operator in the driver's seat with the ability to take over control of the car instantaneously. There are several cars currently on the market with automatic braking and lane holding technologies, neither of which have been deemed to violate road laws.

"Safety has been our first priority in this project," wrote Thurn. "Our cars are never unmanned. We always have a trained safety driver behind the wheel who can take over as easily as one disengages cruise control. And we also have a trained software operator in the passenger seat to monitor the software."