RICHMOND, JAN. 19 -- Virginia Sen. Charles L. Waddell (D-Loudoun) is planning to introduce legislation this week that would allow a private development partnership to build an $80 million, 17-mile extension of the Dulles Toll Road from the airport to Leesburg.

If the legislation passes and the developers satisfy a battery of technical and legal tests required by the state, the highway would be one of the few privately owned, for-profit roads in the nation.

The road would extend from Dulles International Airport west to Leesburg. Transportation planners say it would somewhat relieve traffic on Rtes. 7 and 28 in Loudoun and western Fairfax counties. At rush hour, those roads are among the most congested in Northern Virginia.

In addition, an extended toll road would likely hasten the already considerable development under way in eastern Loudoun, which has become a focal point for intensive residential and commercial construction in recent years.

If the legislation is successful, "we will start immediately down the path" toward building the road, said John D. Miller, president of Municipal Development Corp., a New York-based developer. Municipal Development Corp. and Parsons, Brinckerhoff, Quade & Douglas, a large construction and design firm also based in New York, are principals in a partnership that has proposed building and operating the toll road.

Waddell could not be reached for comment today. The chief lobbyist for the bill, Alexandria lawyer William Thomas, was also unavailable.

Several complex issues are not yet resolved in the legislation. For example, the bill would not give the developers the power of eminent domain, under which the state can seize land for public purposes such as road-building. That means the developers lack a sure-fire way to establish the right-of-way for the road.

It also has not been established whether the developers would operate the toll road or whether they would sell or lease it to the state after construction was finished.

Moreover, neither Loudoun County nor Leesburg has given the project its unbridled support.

The toll for a commuter traveling from Leesburg to the Capital Beltway, where the toll road ends in the east, has not been determined. Some officials estimate the price at $1.50 to $2 each way.

Even if the bill passes, it would not necessarily mean the road would be built with private funds, according to Transportation Secretary Vivian E. Watts. "We support the idea of having the alternative in place," she said. "Until all the details are nailed down, we can't really say" whether the road will be built by the state or the developers.

Whoever finances the road extension, construction is not likely to begin before 1990, Watts said.

In the meantime, a number of environmental studies must be done. And before the extension is built, it is likely the state would have to go forward with plans to add one lane in each direction to the existing toll road.