As almost a half-million school buses this week began the annual ritual that will take them an estimated 4.2 billion miles by the time school lets out next spring, federal authorities have concluded that equipping them with seat belts is an unnecessary expense.

The National Highway Traffic Safety Administration (NHTSA) rejected a petition by safety advocates who sought a federal mandate to require school bus seat belts.

“We care deeply about schoolchildren and feel that the steps we’ve taken are in the best interests of safety,” said Ronald Medford, deputy administrator at NHTSA.

The petitioners, led by the Center for Auto Safety and the National Coalition for School Bus Safety, said they were disappointed but not surprised by the decision.

“It just confirms the long history of NHTSA in opposition to child restraints in school buses,” said Arthur Yeager of the school bus coalition. “There is a certain hypocrisy in their supporting seat belts in virtually every other type of vehicle under their control except for school buses.”

NHSTA explained its decision in the Federal Register on Thursday.

The agency said that large school buses are among the safest modes of transportation, with a fatality rate six times lower that passenger cars. It said that, on average, 19 schoolchildren die each year in bus-related accidents, five of them while aboard buses and 14 in bus loading zones.

NHTSA said that typically those who die in a bus are killed by impact with another vehicle or object and that “in such circumstances seat belts will not be effective in preventing the fatality.”

Small school buses — those weighing under 10,000 pounds — are required to have shoulder-lap belts. But federal safety experts view larger buses as more safe because of their design. Their high-back seats are padded front and back, and the distance between the rows of seats is kept tight. In the event of a front-impact crash, riders will be projected forwarded into a cushioned seat back that is designed to collapse under their weight and absorb the impact.

In Thursday’s Federal Register, NHTSA cited its 2002 report to Congress, which said that shoulder-lap belts are effective in reducing school bus fatalities, but the addition of the belts “would increase capital costs.”

NHTSA estimated equipping each bench-style set would cost between $375 and $600, a total of between $5,485 and $7,346 for each large bus.

“The benefits would be achieved at a cost of between $23 [million] and $36 million per equivalent life saved,” NHTSA said. Rather than face a federal mandate, NHTSA said state and local governments should be left to decide whether to spend the money. Texas and California require school bus belts.

Yeager challenged the NHTSA cost-benefit analysis, saying that in many parts of the country school buses are used year-round for non-school activities.

He also said that the absence of seat belts contributes to school bus accidents and to the death of children killed in the loading zone.

“There are accident after accident where we can document that the cause has been [school bus] driver distraction,” Yeager said. “More kids are killed when their own school bus drives over them than by other drivers. Some of those kids are killed because the driver is distracted by kids jumping up and down on the bus.”

Federal experts say school bus drivers have a tough time enforcing seat belt rules in the morning, when tight schedules must be met. But they say the rules can be more effective in the afternoon when the drivers can refuse to head for homes unless all belts are buckled.

Yeager said that by the time children reach school age they are comfortable with the seat belt culture.

“From their first ride home from the hospital, they have been secured by a restraining device,” he said. “The very first time kids ever experience a ride without a seat belt is when they get on the school bus.”