DEATSVILLE, Ala. — Smaller school buses will have to be equipped with lap-and-shoulder seat belts for the first time under a federal rule drafted following the deaths of four Alabama students on a school bus that nose-dived off an overpass.

Larger buses also will have higher seat backs under the new policy, announced yesterday. That design change is supposed to keep older, heavier students from being thrown over the seats in a collision.

The seat belts will only have to be installed in new buses weighing 5 tons or less, and the requirement will not take effect until 2011. These smaller school buses are already required to have lap belts, but not the safer harness-style belts. There is no seat belt requirement for larger buses.

U.S. Transportation Secretary Mary Peters said she stopped short of requiring seat belts for larger buses because that could limit the number of children who can fit into seats, forcing some children to travel in ways that aren't as safe as school buses.

School districts sometimes expect as many as three younger children to share a bus seat, but if there are only two belts per seat then fewer children can ride the bus.

"We wanted to make sure that any measures we put forth don't needlessly limit the capacity of the buses and then force that school or that school district to have more children walking, riding with parents, biking, etcetera," Peters told the Associated Press in an interview.

At Pine Level Elementary School in Deatsville, about 20 miles north of Montgomery, Gov. Bob Riley and deputy U.S. transportation secretary Thomas Barrett climbed aboard a bus with 12 schoolchildren who have been using the belts in a pilot project.

That started after four Lee High School teens were killed in Huntsville on Nov. 20, 2006, when their school bus went over a highway overpass rail and plunged to a street below, with dozens of students flung from seats.

"We asked the question at that time — would it have been safer if the students on the Huntsville bus had had seat belts?" Riley said. "We were amazed to find out that no one knew."

A bus driver in the pilot project, George Caudle, noted that keeping children from unclicking the belts may be a problem. He said he doesn't start the bus until all are strapped in, but sometimes after he starts driving it's not long before he hears belts being clicked off.

Jacob Chandler, 12, said he was worried about the belts.

"If you were to flip over in a pond, the little ones might not be able to get the belt off and they might drown," he said.

But Monessia Smith, another 12-year-old, said she supports them.

"They're good. It keeps kids safe," she said.

Schools buy about 2,500 of the smaller school buses a year, the Transportation Department said. The buses seat about 16 to 20 students. Larger buses carry more than 50 students.

The Transportation Department estimates it will cost about $6.1 million a year to equip new smaller buses with the three-point seat belts and higher seat backs, and $3.6 million a year to equip new larger buses with higher seat backs.

The rule gives schools the option of using federal highway safety funds to help pay for retrofitting buses with seat belts, in addition to other money already available through the National Highway Traffic Safety Administration.

Jonathan Adkins, a spokesman for the Governors' Highway Safety Association, objected to the use of additional federal funds for school bus seat belts.

"Federal highway safety money is very limited and using that money to install seat belts on school buses isn't supported by crash data," Adkins said.

"School buses are already an incredibly safe mode of transportation. We are more concerned about the areas surrounding schools and bus stops. States should not be pressured on this issue."