A perfectly safe car — just not in the U.S.

"The bumper to absorb energy to reduce the risk of injury is not the kind of bumper system you would design to help protect the vehicle," Vondale said.

Medford said NHTSA's test to make sure cars are safe for unbelted occupants is important in the U.S. market, where people who weren't wearing seat belts make up 45 percent of all traffic fatalities.

"The data that we have really drives the direction and the nature of the standards we develop," he said.

But car makers grumble that NHTSA's requirement makes cars less safe for belted occupants, since protecting people without seat belts requires more powerful air bags and other changes.

"We would much prefer to design a vehicle that provides good protection for unbelted occupants but is tuned specifically to protect occupants that are doing what they're supposed to be doing," Vondale said.

He said 40 countries, including the U.S., have signed on to a 10-year-old effort to harmonize regulations. The group has agreed to frameworks for global regulations governing door locks, rear head restraints and electronic stability control. Next, the group will consider pedestrian protection.

But efforts to match lighting standards, for example, were shelved after Japan nixed a regulation requiring daytime running lights and Europe refused to allow rear turn signals to blink red instead of amber, Vondale said.

"It's very difficult for governments that have had a specific regulation on their books for many years to suddenly decide to change," Vondale said. But he said the short-term cost of redesigning vehicles for the global market would be more than made up by the long-term savings of harmonization.

"We think it's very important to have a single set of requirements that can be enacted globally," he said.

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