MORRISVILLE, N.C. — U.S. Secretary of Transportation Mary Peters proposed new rules Monday to improve the safety of school bus seats and expand the use of shoulder belts, but declined to order that all new buses include seat belts.

Peters rode a packed school bus to Morrisville Elementary School, among the first schools in the country to equip some of its new buses with seat belts, then said she wants to increase the height of seat backs on all school buses from 20 inches to 24 inches to help protect children during accidents. Peters also proposed a new requirement for short school buses — the style more prone to rollover accidents — to begin using shoulder straps. For longer buses, however, she instead proposed allowing states the option of using federal highway safety funds to purchase new buses with seat belts.

But she didn't promise that any new money would be added to those funds to help cover the costs. "We want school districts to make that decision," said Peters, noting that smaller buses don't carry as many students. "They'll make the decision about how to protect the most children within their areas."

A new bus with seat belts costs about $10,000 more than one without the devices, said Derek Graham, a transportation services official for North Carolina schools. North Carolina puts about 800 new buses on the road each year, meaning the seat belt buses would cost the state an additional $8 million each year.

The country has about 474,000 school buses, Peters said.

School districts would have three years to begin having the small buses, which already use the lap belts, equipped with the shoulder restraints. The districts would have to begin using the taller seat backs on new buses one year after the rules are approved.