By JOHN MARKOFF

MOUNTAIN VIEW, California - Google has been working in secret, though in plain view of other drivers, on vehicles that can drive themselves, using artificial-intelligence software that can sense anything near the car and mimic the decisions made by a human driver. With someone behind the wheel to take control if something goes awry and a technician to monitor the navigation system, seven test cars have driven 1,600 kilometers without human intervention and more than 225,000 kilometers with only occasional human control.

One even drove itself down Lombard Street in San Francisco, one of the steepest and curviest streets in America. The only accident, engineers said, was when one Google car was rear-ended while stopped at a traffic light.

Autonomous cars are years from mass production, but technologists who have long dreamed of them believe that they can transform society as profoundly as the Internet has. Robot drivers react faster than humans, have 360-degree perception and do not get distracted, sleepy or intoxicated, the engineers argue.

They speak in terms of lives saved and injuries avoided - more than 37,000 people died in car accidents in the United States in 2008. The engineers say the technology could double the capacity of roads by allowing cars to drive more safely while closer together.

Because the robot cars would eventually be less likely to crash, they could be built lighter, reducing fuel consumption. The Google project is the brainchild of Sebastian Thrun, the director of the Stanford Artificial Intelligence Laboratory and the co-inventor at Google of the Street View mapping service.

Sounding like an evangelist when he spoke of robot cars, Dr. Thrun envisioned cars that would not need anyone behind the wheel and could be summoned electronically so that people could share them.

That would mean fewer cars, reducing the need for parking spaces, which consume valuable land. And, of course, the cars could save humans from themselves. “Can we text twice as much while driving, without the guilt?”

Dr. Thrun said in a recent talk. “Yes, we can, if only cars will drive themselves.” The Google research program using artificial intelligence to revolutionize the automobile is proof that the company’s ambitions reach beyond the search engine business.

During a recent half-hour drive beginning on Google’s campus south of San Francisco, a Prius equipped with a variety of sensors and following a GPSroute programmed into the GPS route nimbly sped up in the entrance lane and merged into fast-moving traffic on Highway 101, the freeway through Silicon Valley. It drove at the speed limit, which it knew because the limit for every road is in its database, and left the freeway several exits later. Christopher Urmson, a robotics scientist, was behind the wheel but not using it.

To gain control of the car he had to do one of three things: hit a red button near his right hand, touch the brake or turn the steering wheel. The advent of autonomous vehicles poses thorny legal issues, the Google researchers acknowledged.

Under current law, a human must be in control of a car , but what does that mean if the human is not really paying attention , figuring that the robot is driving more safely than he would? I n the event of an accident, who would be liable ? the person behind the wheel or the maker of the software? “The technology is ahead of the law in many areas,” said Bernard Lu, senior staff counsel for the California Department of Motor Vehicles.