US safety groups have said that a proposed regulation to require stronger vehicle roofs was inadequate for protecting motorists in rollover crashes, which kill more than 10,000 people a year.

On the other side, according to The Associated Press (AP), automakers said the government underestimated the design changes that would be needed to upgrade their fleets and urged regulators to give them more time.

AP noted that the National Highway Traffic Safety Administration proposed the regulation in August requiring roofs to handle direct pressure of 2.5 times the vehicle weight, an increase from the current rule of 1.5 times the weight.

For the first time, it takes into account large sport utility vehicles and pickups such as the Ford Expedition and Dodge Ram while seeking ways to protect occupants in the future through improved seat belt technology.

But safety groups reportedly said about 70% of vehicles already comply with the standard and said the rule was developed arbitrarily, with too much focus on how much it would cost the motor industry instead of designing a regulation to save more lives.

"The agency should go back to the drawing board and develop a far more stringent and effective test," Joan Claybrook, president of watchdog group Public Citizen, told The Associated Press. She reportedly called the proposal "grossly inadequate" and the result of "junk science."

NHTSA spokesman Rae Tyson reportedly said the proposal had been in development since 2001 and involved the "best minds in government" who thoroughly examined an upgrade to the rule, which has been in place since the 1970s.

"It's not junk science. It's very well-reasoned science," Tyson told AP.

Under the proposal, vehicles would likely need to come into compliance by September 2009, but automakers said a much larger proportion of vehicles would have to be redesigned than estimated by NHTSA and asked that the rule be phased in.

According to The Associated Press, Bob Lange, General Motors' top safety official, wrote that the changes "are quite significant and will consume large amounts of engineering, manufacturing, and capital resources that are not now comprehended in our product cycle plans."

The Alliance for Automobile Manufacturers, a trade group representing nine automakers, reportedly said the proposal would require more head room in the vehicles, leading to potential unintended safety consequences, such as making vehicles more top heavy.

AP added that outside groups have also raised concern about the proposal's language that could prevent plaintiffs from suing automakers if they fail to have roof crush standards stronger than the federal rules. The provision is supported by automakers.

AP said two senators wrote to NHTSA acting administrator Jacqueline Glassman last week to ask about the proposal. "Congress mandated that NHTSA establish standards to 'reduce vehicle rollover crashes and mitigate deaths and injuries associated with such crashes.' It seems to us that this end will not be served by the new proposed Rule," they wrote, according to the news agency.

The provision would be part of the agency's deliberations, Tyson told The Associated Press.

NHTSA reportedly has estimated the rule change will cost the industry US$88 million to $95 million a year and save 13 to 44 lives per year, and could prevent 500 to 800 injuries a year.