Google on Tuesday told U.S. senators that the federal government must fast-track regulations to allow self-driving cars on U.S. roads so America can stay ahead of China, Europe and Japan.

"They're hot on our heels," said Chris Urmson, director of Google's driverless car unit.

A "growing patchwork" of state regulations threatens the viability of robotic cars in the U.S., Urmson told the Senate Commerce Committee. Without unified rules, the vehicles couldn't travel between states, creating a "challenge in delivering the technology broadly."

"Where we're most concerned is bringing this to market," Urmson said. "In the past two years, 23 states have introduced 53 pieces of legislation that affect self-driving cars — all of which include different approaches and concepts."

Google wants nationwide rules allowing fully autonomous cars with no steering wheels or brake pedals, because it believes the computer will always make the best decisions and humans could create safety issues by trying to override cars' choices. Congress could give the National Highway Traffic Safety Administration power to approve self-driving car innovations.

In California, Google faces pending rules barring fully autonomous vehicles. Draft regulations from the Department of Motor Vehicles would require human-override capability in the event of a technology failure or another emergency. Those rules are likely to be adopted, but federal regulations from the traffic safety administration would trump California rules, said DMV spokeswoman Jessica Gonzalez.

Google's main foe on self-driving cars has taken aim at the company's plan to put completely autonomous cars onto the streets. In Google's own testing of self-driving cars, drivers have had to take control over and over again, noted John Simpson, consumer advocate for Consumer Watchdog. "Two hundred seventy-two times essentially the computer said, 'I can't handle this,' and turned over control. The test driver felt compelled to intervene 69 times. The DMV has got it exactly right."

The traffic safety administration issued a preliminary report this month that said driverless cars without steering wheels or brake pedals do not meet federal regulations and would require an exemption. In a letter to Google last month, the agency suggested that Google might "wish to reconsider its view that a pedal may never be needed in any circumstance, and that there is not a risk of harm associated with a pedal's absence."

Industry players, including Google, believe people lose their ability to effectively override automated systems once they get comfortable in self-driving cars. "It's very challenging to rely on human drivers to take over," said Andrew Ng, chief scientist for Baidu, known as "China's Google," which plans to put commercial robot cars on Chinese roads by 2018. "Most people acclimatize too quickly to autonomous driving. A trained safety driver will maintain vigilance. Normal people like you and me, it's very difficult to have people maintain vigilance for hours."

Google's Urmson made that point in the Senate hearing. "At some point the automation technologies are just so good that people over-trust it, even when they're told they shouldn't," Urmson said. "This is … why we're taking that leap toward fully self-driving vehicles."

Consumer Watchdog has appeared somewhat out in the cold in the California debate over steering wheels and brake pedals, but Google's recent admission that its car had some responsibility in a crash with a bus may change that, the DMV's Gonzalez suggested. In developing draft regulations, the DMV held public hearings, and Consumer Watchdog's safety advocates were "mainly the only ones that we've heard from" in opposition to fully autonomous vehicles, Gonzalez said.

"A lot of times people don't know about the technology enough to be concerned," Gonzalez said. "That recent crash with Google and the bus made people think a little differently. It's the first crash that I think had people realize, 'Oh, maybe the technology isn't quite ready yet.' "

Duke University robotics department director Missy Cummings told senators that the technology holds immense promise, but too-speedy rollout could lead to tragedies that would hamper adoption. "I am decidedly less optimistic about what I perceive to be a rush to field systems that are absolutely not ready for widespread deployment, and certainly not ready for humans to be completely taken out of the driver's seat," Cummings said.

Contact Ethan Baron at 408-920-5011 or follow him at Twitter.com/ethanbaron.