The Fairfax County School Board, which operates the nation's largest school bus fleet, is preparing to end a brief experiment and drop the use of seat belts on its vehicles, which transport 87,000 children per day.

The move follows the release of a National Transportation Safety Board study in March that concluded that lap belts do not improve safety and that they may detract from it in some cases.

Superintendent Robert R. Spillane plans to ask the School Board to vote Thursday to buy its fall order of 186 full-size buses without seat belts, a policy change that would require approval of the Board of Supervisors, which funds the purchase.

"It just makes a lot of sense not to put those seat belts on," Spillane said yesterday. "It's hard to refute a safety board finding like that."

Spillane said he will not recommend removing the belts from the 263 of the system's 1,100 vehicles that now have them; he added that he will continue to make their use optional.

If Fairfax drops the belts, Montgomery County would be the only area jurisdiction with a policy requiring that newly purchased school buses be equipped with seat belts. Of Montgomery's 793 buses, about a third have seat belts, and their use is optional, officials said.

On smaller buses that transport handicapped children, seat belts are standard equipment in most area jurisdictions.

Efforts nationwide to install seat belts in school buses stemmed from a lobbying campaign by parents who argued that the belts improve safety, help enforce discipline on buses and are consistent with efforts to persuade children to use seat belts in automobiles. New York state requires that all buses purchased after July 1 have belts, and about 150 districts nationwide use buses equipped with seat belts.

But the National Transportation Safety Board study released in March concluded that buses built after 1978, when updated federal safety standards took effect, are "an extremely safe form of transportation." It recommended against installing lap belts.

Federal safety standards require that buses built after 1978 have padded seat backs, higher seat backs and seats spaced closer together, making for a protective "egg carton" effect.

Fairfax County will have 58 buses built before 1978 left in its fleet after its fall order is delivered, school officials said.

The national board study of 43 accidents concluded that passengers would have received "no net benefit from lap belt use." In addition, if lap belt-induced injuries are taken into account, the safety board report said, "the introduction of lap belts would have had a negative effect on these passengers' safety."

Opponents also argue that belts can become weapons in the hands of children, can slow evacuation in an accident and can add $1,000 per vehicle to the bus purchase cost at a time when school system growth is pinching the transportation budget.

The first 70 buses with seat belts arrived in Fairfax County 18 months ago. After four months of use, the school system's own study concluded that there was no national research to show that the belts improved safety and some to indicate that they might be dangerous in certain circumstances. The School Board voted to continue purchasing buses equipped with seat belts until more research was available, but it made use of the belts optional.

There was no accident with injury during the period when the belted buses were being studied, so Fairfax officials relied on national studies for their safety information.

Spillane's latest recommendation to drop the belts is coupled with one to add other types of safety equipment on new buses, including crossing arms to prevent children from walking too close to the front of buses and an eight-lamp warning light system to make buses more visible in traffic.

Spillane hinted in March, when the U.S. transportation board study was released, that he would recommend against installing seat belts in newly purchased school buses. And School Board members said they had heard no objections from the activists who earlier had lobbied for seat belts. They said they doubted that their vote will be controversial in light of that study.

Montgomery is in the process of buying 230 buses with seat belts, expected to be delivered in September, as part of a speeded-up campaign to replace its older vehicles, said William C. Westcoat, school system automotive maintenance supervisor.