A plan to create a special tax district to let developers pay for one of Northern Virginia's most sorely needed road improvements became a reality yesterday.

The Loudoun County Board of Supervisors, mirroring action taken by Fairfax County lawmakers two weeks ago, approved the formation of a commission that will levy a special tax on the Rte. 28 landowners, who would profit most from widening the highway near Dulles International Airport. The airport straddles the Fairfax-Loudoun boundary.

The revenue raised from a 20-year surtax of 20 cents per $100 of assessed value is expected to fund about 80 percent of the $392 million cost of widening the congested two-lane highway; the state will finance the rest.

Yesterday's action cleared another legal and legislative obstacle from a long course. Before any asphalt is laid, however, at least one further blockade must be removed. In its session starting next month, the Virginia General Assembly would have to approve measures giving local governments more authority to borrow large sums of money.

The Virginia Supreme Court last month cast doubt over the planned method of financing Rte. 28 expansion by declaring unconstitutional the use of so-called pledge bonds -- which "pledge" gasoline taxes and other expected highway revenue to pay off debt.

Curtis M. Coward, an attorney for the Rte. 28 landowners, said he is optimistic that the legislature will act this winter to let work begin.

As Dulles Airport has blossomed, Rte. 28 has emerged as one of the leading growth and high-technology corridors on the East Coast. Relying on traditional state Department of Transportation funding to widen the road would take several years and threaten the area's prosperity, Loudoun and Fairfax officials fear.

"We are at a time that I think we will all look back upon as a historic moment," Coward said at yesterday's meeting.

If the General Assembly acts favorably, Rte. 28 construction should begin by early 1989, Coward said.