Companies from Silicon Valley to Detroit to Germany are developing cars that park, steer and even drive themselves. Now the federal agency for traffic safety has said it wants to come along for the ride.
The Department of Transportation made its first formal policy statement on autonomous vehicles. In a nonbinding recommendation to the states, it said that driverless cars should not yet be allowed, except for testing. It said, however, that semiautonomous features, such as cars that keep themselves centered in lanes and adjust their speed based on the location of the car ahead, could save lives.
The statement, from the department's highway safety agency, comes as companies, led by Google, have made significant technological strides in making cars that drive themselves but still face daunting legal, regulatory and cultural hurdles before the cars are widely available. It is the latest example of the tension between technological innovation and regulation, which move at very different speeds.
It is also a time of rapid change, and some anxiety, about autonomous systems in general. The transportation department is struggling, for instance, to determine how to regulate drone aircraft.
The statement detailed the benefits of self-driving and semiautonomous cars, which analysts said was a recognition by government officials that it had no choice but to keep up with the advancing technology in this area, which falls on a continuum from cruise control to full automation.
"It's not that they're trying to put the brakes on it," said Richard Wallace, director of transportation systems analysis at the Center for Automotive Research in Ann Arbor, Mich. "They're trying to get out in front of it."
Still, the highway safety agency was careful to address the tension between technology and regulation.
"Any potential regulatory action must appropriately balance the need to ensure motor vehicle safety with the flexibility to innovate," it said.
Even though technology companies like Google generally fear that innovation far outpaces regulation and risks being stifled by it, it has a different approach with cars than with software or cellphones because cars have been heavily regulated for decades, said Ryan Calo, a law professor at the University of Washington who co-founded the Legal Aspects of Autonomous Driving center at Stanford.
"We want to have some experimentation in the states to see what works, but it's nice to have federal experts helping out, as long as they don't take it too far," he said.
Autonomous cars could increase safety because they are not subject to human error like disobeying traffic laws and falling asleep at the wheel, according to analysts, car companies and the transportation department. They could also offer mobility to people who cannot drive, such as the disabled or elderly.
Driverless cars could "change our lives, give us more green space, mobility, fewer hours wasted," Larry Page, Google's chief executive, said this month. "The average American spends 50 minutes commuting. Imagine if you got that back."
Still, many Americans are dubious about automated driving, according to a poll by the Auto Alliance, a Washington trade group that represents 12 of the largest carmakers selling vehicles in the United States. For instance, 81 percent said they were concerned that computer hackers could take control of an automated vehicle.
Even if automated cars were safer, people would worry about the lack of human judgment, Calo said.
"The first time that a driverless vehicle swerves to avoid a shopping cart and hits a stroller, someone's going to write, 'Robot car kills baby to save groceries,'" he said. "It's those kinds of reasons you want to make sure this stuff is fully tested."
Yet most people will have the option of buying a car that is part robot in some sense next time they visit a dealership. Vehicles ranging from German luxury cars to mass-market US sedans are now equipped with automated safety systems that rely on computer processors, software and sensors.
Future models from Mercedes-Benz have radar systems that brake a car in the event of an impending collision, stay in its proper lane around curves and sense when a driver is fatigued. Ford Motor Co.'s midsize Fusion sedan has a lane-assist system that alerts drivers when they stray on the roadway. Many cars come with adaptive cruise control that automatically cuts the speed when the distance between vehicles gets too close.
Greg Martin, a spokesman for General Motors, the largest U.S. automaker, said, "G.M. has been working hard on autonomous vehicle technologies because we believe in its safety potential."
Google has gone the furthest, equipping Lexuses and Toyota Priuses with technology that drives the car without human intervention. Although Google still requires people to sit in the driver's seat, employees use the cars to commute the 40 miles between San Francisco and Mountain View, Calif., its headquarters, and have driven the curves of California State Route 1, a treacherous road that overhangs the Pacific Ocean. The cars have driven more than half a million miles, according to the company.
The government's statement would likely not slow development of automated safety systems, but regulators expressed discomfort with driverless cars like Google's, which some people predict will be commercially available in less than a decade.
"Self-driving vehicle technology is not yet at the stage of sophistication or demonstrated safety capability that it should be authorized for use by members of the public for general driving purposes," the document said.
Leslie Miller, a Google spokeswoman, did not respond directly to the statement. She said Thursday, "We are introducing autonomous vehicle technology to improve people's lives by making driving safer, more enjoyable and more efficient."
People in the driverless technology industry said the agency's caution about self-driving cars was reasonable.
"You can't write regulations for something that is just getting into the prototype stage," said Richard Bishop, who represents the Association of Unmanned Vehicle Systems International, a Washington trade association.
It is up to state and local governments to decide whether autonomous or semiautonomous cars are allowed on public roads. States including California, Nevada and Florida have already legalized driverless cars.
They are not explicitly illegal in other states, because there is no law that says cars must have drivers.
The National Highway Traffic Safety Administration, which issued the policy statement Thursday, has jurisdiction over the vehicles themselves.
Driverless car technology is not advanced enough to develop safety standards, according to the statement, which defined four levels of autonomous vehicles. It said it was beginning a four-year research project on how to safely use automation, including studies of how humans interact with the cars, the reliability of the technology and risks such as cyberattacks.
The agency offered recommendations for the states, including requiring drivers to get special licenses to operate autonomous vehicles, cars to have a button within easy reach that returns control to the driver and companies to report detailed data on accidents.