GOOGLE Inc is road-testing cars that steer, stop and start without a human driver, the company says.

The goal is to "help prevent traffic accidents, free up people's time and reduce carbon emissions" through car sharing and the new "highway trains of tomorrow," project leader Sebastian Thrun wrote on Google's corporate blog.

The cars are never unmanned, Thrun wrote. He said a backup driver is always behind the wheel to monitor the software.

They have traveled a total of 140,000 miles on major California roads, navigating San Francisco's curvy Lombard Street and Los Angeles' Hollywood Boulevard without much human intervention, according to the blog.

The cars know speed limits, traffic patterns and road maps, Thrun's posting says. They use video cameras, radar sensors and lasers to detect other cars.

Google has not revealed how it hopes to profit from the research.

The company is flush with cash, though, and pushing numerous projects that are unrelated to its core business, said Rob Enderle, principal analyst with the Enderle Group in San Jose.

"The word 'focus' is a word Google has never learned," Enderle said, pointing to projects involving electricity distribution, vehicle design and artificial intelligence.