The statement 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 highway safety agency was careful to address the tension. "Any potential regulatory action must appropriately balance the need to ensure motor vehicle safety with the flexibility to innovate," it said.
Many Americans are dubious about automated driving, according to a poll by the Auto Alliance, a Washington trade group. For instance, 81 percent said they were concerned that computer hackers could take control of an automated vehicle.
It is up to state and local governments to decide whether autonomous or semiautonomous cars are allowed on public roads. States such as 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.
But expect fast change, says Google CEO and co-founder Sergey Brin. "You can count on one hand the number of years until ordinary people can experience this," he said last September, when California Gov. Jerry Brown signed a law allowing self-driving vehicles.
Google has more than 500,000 miles on Toyota Prius and Lexus RX 450h vehicles that use radar, lasers, cameras and computers to autonomously go from point A to B without whacking other traffic.
"The Google car is the most automated," the safety agency's Friedman said, but even it is not "what I jokingly call 'The Jetsons Car,' where you sit back and enjoy the ride, (and) your job is just to tell the car where to go."
NHTSA defined levels of vehicle automation from 0 to 4. The Jetsons' car would be a 4. Google's are Level 3, meaning a driver must be at the wheel, able to take back control. "Level 3 is truly in the testing phase. How does the car warn the driver of the need to take over? How much warning does it give?" Friedman said.
In a level 2 car, many on sale now, the driver is in overall control, but two or more automated systems such as adaptive cruise control and lane centering can work together to independently correct the vehicle.
At level 1, the driver's in charge, although function-specific systems, such as stability control (now federally required) may assist. NHTSA says that its research into automated systems may result in more being required.
Level 0? "That's what most of us have driven most of our lives. You're the only thing responsible for all the core functions," Friedman said.
The government defines levels of vehicle automation from 0 to 4
Level 4 • The "Jetsons' car," where driver sits back, tells car where to go
Level 3 • The "Google car," where driver must be at the wheel, able to take back control
Level 2 • On sale now, where driver is in control, but automated systems such as lane centering can make corrections
Level 1 • Readily available, where driver is in charge, although functions such as stability control may assist
Level 0 • What most of us have driven most of our lives