A federal appeals court yesterday ordered the government to scrap a new rule extending the number of hours truck drivers can spend on the road in a day, saying it was imposed without consideration of its impact on trucker health.

The decision was a victory for several consumer watchdog groups that sued the Federal Motor Carrier Safety Administration, an agency of the Department of Transportation, claiming the longer hours would worsen driver fatigue and lead to more highway deaths and injuries.

The new rule, issued in January, increased to 11 hours from 10 the amount of time a trucker can drive without a break, even though the government conceded that "studies show . . . that performance begins to degrade after the 8th hour on duty and increases geometrically during the 10th and 11th hours," according to court documents.

"This is a serious issue. These drivers are driving massive commercial trucks, and if they're tired they're endangering not only their own safety but everyone sharing the road with them," said Bonnie Robin-Vergeer, a lawyer for Public Citizen who argued the case in U.S. District Court in Washington.

A three-judge panel of the U.S. Court of Appeals for the D.C. Circuit found the new "hours of service" rule to be "arbitrary and capricious," and said several aspects of the case "raise troubling concerns about the decisionmaking process" at the government agency, in the decision filed on behalf of the panel by U.S. Appeals Court Judge David B. Sentelle.

Long-distance truckers are under increasing pressure to move goods quickly in an age of just-in-time inventories, and the job is one of the most hazardous in the United States. According to the Bureau of Labor Statistics, truckers suffered seven injuries or illnesses per 100 workers in 2002, far above the 5.3-per-100 rate for private industry as a whole.

Driver fatigue likely contributes to 40 percent of all accidents involving heavy trucks and almost 60 percent of single-vehicle truck crashes, the National Transportation Safety Board said in a 1995 study.

Last year, 4,942 people were killed in accidents involving big trucks, according to government statistics. Only about 710 of those deaths involved occupants of the trucks, showing how deadly such accidents are to other motorists, Robin-Vergeer said.

The government has 45 days to decide whether to appeal yesterday's ruling, and the extended hours will remain in effect in the meantime, Department of Transportation spokesman Robert Johnson said. The judges left open the possibility that the agency could go back and address trucker health in justifying the new rule.

"We think the rule is a good one and are now in the process of taking a look at how to respond given the court's decision today," Johnson said.

The American Trucking Associations, which supports the extended hours of operation, sent a bulletin to its members yesterday emphasizing that the rule could remain in effect as long as 90 days if the government requests a stay of the decision. "ATA believes that switching back and forth between the old and new rules would be confusing to the point of adversely affecting highway safety," the association said in its bulletin.

Association spokesman Mike Russell said the group supports the new extended hours because the rule also increases the amount of rest required between shifts, to 10 hours from eight.

But safety advocates argue that the extra two hours is not enough, partly because truckers with sleeper cabs can split their rest into two shifts.