A federal committee on Monday rejected the idea of requiring seat belts on the nation's more than 300,000 larger school buses, concluding that it would cost $40 million and save an average of just one life a year.

"The overall potential benefits of requiring seat belts in school buses is insufficient to justify a federal standard mandating installation," said a committee of the National Research Council after an 18-month study. The NRC was set up by the National Academy of Sciences.The committee said, however, that some steps should be taken to improve what transportation officials say already is one of the safest public conveyances, including raised seat backs and programs to prevent accidents while children are getting on and off the buses.

The group estimated that if all school buses were equipped with seat belts for each passenger and half the students used them, it might save one life and prevent several dozen serious injuries a year. It would cost $40 million a year to equip and maintain all larger school buses with passenger seat belts, the study said.

Raising seat-back heights from a minimum of 20 inches to 24 inches could save two to three lives, prevent up to 95 serious injuries and cost about $6 million a year, the report said.

The National Highway Traffic Safety Administration does not require seat belts in buses rated at more than 10,000 pounds gross weight that typically carry more than 16 passengers. Smaller, van-like buses must have belts, however.

The study, ordered by Congress and funded by the Department of Transportation, recommended that all states prohibit children from standing on school buses when they are in motion. Some states allow school buses to carry more children than there are seats.

The committee, which included a wide range of safety and transportation experts, also recommended that states, local school districts and private contractors stop operating school buses made before 1977 because they do not comply with current safety standards.

Buses built after 1977 have extra padding in seats, protected fuel tanks and other modifications to make them less deadly in crashes.