A narrow dirt road that raises dust in the summer and is sprinkled with potholes in the winter has become the key that could unlock land in Loudoun County west of Washington-Dulles International Airport to rapid development.

The road in question is Route 643, a scenic byway that starts in Leesburg as a hardtop road, then extends southeast before turning into a gravel road and petering out in fields about one mile west of the airport.

Developers are promoting the idea of easing the traffic burden on Route 7 by turning Route 643 into a four-lane toll road that would connect with the Dulles Airport Access Road's toll lanes at Route 28.

The proposal surfaced last summer when a citizens task force drew up a land-use study for Dulles North, the area directly west and north of the airport property. Loudoun County commuters heading to work in the metropolitan area travel via Route 7 and Route 28 to meet the toll lanes. The proposed highway would bypass these roads.

The proposal for upgrading Route 643 initially was dismissed as a planner's pipe dream. But it began to make sense as developers began filing ambitious plans for eastern Loudoun, explained county Planning Director Frederick P. D. Carr.

"We used to call it the Calderon Causeway after Richard Calderon, the planner who dreamed it up, but then we took another look at the map," Carr said, tracing his finger along 643's path. It runs directly from Leesburg to the existing toll lanes.

For the proposal to advance, county officials would have to adopt it, an action they are expected to take this summer. There then would be time for public comment before a board of supervisors decision.

Traffic projections suggest that developments proposed along Route 7 promise to turn the road into a parking lot despite plans to upgrade it from a four-lane to a six-lane highway with interchanges and limited access.

Rezoning applications for Xerox Corp.'s proposed corporate park and homes north of Route 7 near Leesburg, Ashburn Village I and II and Ashburn Farms would add at least 11,261 homes to eastern Loudoun between Route 28 and Leesburg. The same plans propose sufficient commercial and industrial development for 18,790 jobs, which would double the county's current employment base.

In addition, plans are on file to build another 2,176 homes in Leesburg, enough to increase the town's population by 50.9 percent in the next 10 years. Leesburg also is considering applications for two major industrial parks on 775 acres near the municipal airport.

All this traffic would funnel onto Route 7. Although a Loudoun County study indicates that the highway from Leesburg to Route 28 can cope today with its average traffic count of 23,900 vehicles daily, members of the county planning staff predict that Route 7 traffic will triple by the year 2010, and that's using conservative estimates.

These figures seem to have convinced the planners, developers and politicians alike that Route 643 is the answer to transportation woes. "It would be another lifeline to the east," Leesburg Planning Director Martha Mason Semmes said.

The idea already has won unusual support, according to Arthur J. Smith, Loudoun's transportation planner. "For a project that is less than a year old, it's an extraordinarily vigorous baby," Smith said.

The beauty of the project is that it could bypass the eternal wait for Virginia Department of Highways and Transportation funding, according to Carr. Improvements to Route 643 are not even listed in the county's six-year road plan. Secondary-road funding for Loudoun County is $8.5 million a year, scarcely enough to build the two bridges over creeks that the proposed highway would have to include.

Backing from the development community, however, could move the highway from the drawing board onto the ground by the early 1990s, according to county Supervisor Ann B. Kavanagh, who represents this area.

The road passes through land held by a handful of large property owners, and Kavanagh is confident that they will make major donations toward the road when the landowners seek rezoning. "They are landlocked for all practical purposes, and if they want to open land to development, they will have to help pay for the highway," she explained.

John A. Andrews, a key landowner along Route 643, is an enthusiastic supporter of the road-expansion plan. "I think it's a great plan," he said. "Our engineers already are considering alternative alignments." Andrews, president of Herndon Concrete and vice president of InterGate Co. Inc., a land development firm, is a partner in the 1,000-acre planned residential community proposed for Ashburn Farms south of Ashburn Village. He also is preparing plans for another residential community behind Broad Run High School. Both properties are along Route 643.

Using the proffer system -- under which developers trade road or other public improvements or funding for favorable zoning decisions -- the county planners and politicians are banking on accumulating cash donations and rights of way to build the four-lane highway through to the Dulles Airport Access Road and toll lanes. Already Alan I. Kay, developer of 5,461 units at the proposed community in Ashburn, has proffered $1.4 million to a highway trust fund that could go toward upgrading Route 643.

Leesburg also is receiving donations for its end of the highway. Developers of Exeter, a project in the northeast quandrant of Leesburg, are proffering $438,365 for off-site road improvements. That money also could go toward Route 643, said Semmes, the Leesburg planning director. The town also is seeking 100-foot right-of-way donations from two developers applying for rezoning for industrial parks along Route 643, she said.

To speed funding for the highway further, developers want to make 643 a toll road. "It would be the best way to finance it. Look at the success of the existing toll lanes," Andrews said.

That proposal, however, is running into opposition from the influential state senator for Loudoun, Charles Waddell.

"My senatorial district is the only one in the state of Virginia with a toll road, and I'm not about to endorse another," Waddell said. He is chairman of the Virginia Senate's Transportation Committee, and state legislation is required to build toll roads in Virginia. But Waddell said that if citizens voted for the toll road in a referendum, he would back it.

However an upgrading of Route 643 gets funded, planners are moving ahead on the assumption the work will be done.

Smith said he is negotiating with Xerox Corp. to have it improve Route 659 from Route 7 to the proposed highway. This would feed thousands of cars that the Xerox project is expected to generate away from Route 7.

Kavanagh said that Federal Aviation Administration officials indicated during an informal meeting that they are open to proposals to have Route 643 cut across the Dulles Airport property to join the existing toll lanes. If that proposal fails, 643 would have to join Route 606 and intersect with Route 28 a short distance north of the toll lanes.

And the Loudoun County Board of Supervisors showed its confidence in the future of Route 643 Monday when it voted to build government facilities on 48 acres alongside 643 outside Leesburg.