"The cars uses video cameras mounted on the roof, radar sensors and a laser range finder to 'see' other traffic", said software engineer Sebastian Thrun.
However, during the test drive, they remain manned at all times by a trained driver ready to take control as well as by a software expert, reports the BBC.
In a posting on the company's official blog, Thrun said the self-driven cars had so far covered 140,000 miles on the road.
They have crossed San Francisco's iconic Golden Gate bridge, negotiated the city's famous sloping streets, driven between Google offices, and made it around Lake Tahoe in one piece.
Engineers told the New York Times that the forays onto the highways have been largely incident-free, apart from one bump when the car was reportedly hit from behind at a traffic light.
Routes are pre-planned, mapped first by real drivers, and local police are briefed in advance, Thrun added.
"While this project is very much in the experimental stage, it provides a glimpse of what transportation might look like in the future thanks to advanced computer science. And that future is very exciting," he added.