The Fairfax County Board of Supervisors, eager to relieve the traffic congestion that threatens to cripple the development boom near Dulles International Airport, endorsed a proposal yesterday that would impose a special tax on developers to pay for improvements to that area's main artery.

The special development tax would be used to help finance a $50.5 million project to widen Rte. 28 (Sully Road), a two-lane highway that slices through land planned for some of the most intensive commercial and industrial development in the metropolitan area, including Virginia's Center for Innovative Technology. The tax would have to be authorized by the state legislature.

"Without transportation improvements, the whole area's going to come to a complete stop," said board Vice Chairman Martha V. Pennino (D-Centreville).

"This tax would be exclusively on developers," said Chairman John F. Herrity, a Republican who introduced the plan. "It would not affect the residents out there."

The board voted 8 to 0 to endorse the proposal, with one abstention.

Herrity's proposal recommends imposing a tax of 10 to 20 cents per $100 of assessed value on developers who own land along Rte. 28. The tax would be levied by an authority, similar to a sanitary district, that could float bonds to raise money for the project. Herrity said many developers, frustrated by traffic problems in the region, support the proposal.

The plan to widen Rte. 28 recommends partial state financing, possibly in the form of a loan. The proposal for the special taxing authority and the state assistance would require approval by the Virginia General Assembly, which is to meet in a special session in September to consider strategies for improving transporation statewide.

In another development-related action yesterday, the county board adopted a policy setting strict guidelines for homeowners who band together to sell their neighborhoods to developers. The policy is the board's response to an increasing number of neighborhood sell-outs in fast-developing areas of the county.

The policy curbs the practice when it would lead to land uses that do not conform to the county's master development plan. County officials said that the boom in neighborhood consolidations is reducing the county's residential acreage.

Under the policy, consolidations would be allowed if the homeowners can demonstrate that the proposed use of the land would not create traffic problems and hamper county services in adjacent neighborhoods. Under existing county policy, the board can reject any development proposal that does not conform to the master plan.

Despite general agreement that Rte. 28 should be widened to control traffic problems in western Fairfax and eastern Loudoun County, developers and state, federal and local officials have been deadlocked for about two years over how to finance the widening of the road.

County planners expect that at least 1.5 million square feet of office space will open annually in the next few years along the corridor, just east of Dulles.

Supervisor Audrey Moore (D-Annandale) abstained on the vote, concerned that state support for Herrity's plan would shift scarce state road money from transportation projects planned in her central Fairfax district, including a proposed commuter rail system.

Herrity and county staff members said they are uncertain about how much state money would be required to support the project, but they said they will recommend that state aid be provided in the form of a loan, not a grant.

The proposal asks Loudoun officials to join Fairfax in the plan. Without Loudoun's support, the six-lane portion of Rte. 28 in Fairfax would feed into the two-lane stretch in Loudoun, creating a potentially serious bottleneck at the county line.

Loudoun Supervisor Frank Raflo, a key member of the Loudoun board on development issues, stopped short of endorsing the Herrity plan yesterday.

"If it's fine for Mr. Herrity, that's fine," said Raflo. "But our decisions are made in Loudoun, not Fairfax."