A committee of Northern Virginia's top political leaders has expanded its wish list of transportation projects for the next 20 years to include two new bridges over the Potomac River, a Metro extension into Prince William County and a 60-mile outer beltway crossing the countryside from Stafford County to Loudoun County.

The projects would add an estimated $4.1 billion in spending to a list of new road and transit proposals for the region that now totals $15 billion.

The Northern Virginia Transportation Coordinating Council, a bipartisan group of three dozen elected officials, will consider the new version of the "2020 Plan" tomorrow and hopes it will serve as a blueprint for transportation funding during the General Assembly session that begins next month.

This latest version was approved Friday by the council's executive committee, following heated criticism from developers and business leaders who complained that the original wish list relied too heavily on buses and trains to solve the area's traffic woes.

Supporters argue that the new items would dramatically ease congestion along the heavily traveled Interstate 95 corridor by diverting automobile traffic onto new thoroughfares.

"This is in response to citizen comment and to business community comment," said Fairfax City Mayor John Mason, a member of the executive committee that endorsed the latest proposals. "We clearly saw that we still had a major problem in the plan with the I-95 corridor. It would have been a failing on our part not to be responsible and responsive to that."

But slow-growth activists and dissenting politicians argue that the projects would only worsen traffic problems by further encouraging development in far-flung areas, and they say the money would be better spent elsewhere.

"The original plan had a balance that would have served the region well," said Falls Church Mayor David F. Snyder (I), who was one of three leaders to vote against the outer beltway proposal.

"That balance has been lost. . . . These projects will just encourage more sprawl without adding a lot of value for citizens."

The three major proposals added by the executive committee are:

* The Western Transportation Corridor, costing $1.6 billion. The corridor would start at I-95 in Stafford and pass through Prince William on its way to Loudoun, where it would cross over the Potomac into Maryland. This bypass proposal has been a political hot potato for years, supported by many local politicians in Virginia while attracting vehement opposition from citizen activists.

* A new bridge to connect Interstate 95 with Route 301 in Southern Maryland, with a crossing likely in either Prince William or Stafford. As with the western bypass, this $1 billion proposal likely would run into strong opposition on the other side of the Potomac, where Maryland Gov. Parris N. Glendening (D) and other officials have been cool to talk of new bypasses.

"There are people that believe that if you don't have the bridges in the plan, you let Maryland off the hook," said Kenneth Klinge, chairman of the transportation council. "At least we're attempting to solve our problems."

* A measure pushed by Prince William officials to extend Metro to the area near the wildly popular Potomac Mills discount mall, adding seven miles to the Blue Line, which ends in Springfield and was projected to end in Lorton. The proposal would cost an estimated $1.5 billion.

But Prince William Board of Supervisors Chairman Kathleen K. Seefeldt (D) said the project would be well worth it for commuters in the area. "We could not support a plan that did not show improvement in that corridor," she said. "It's the most heavily used corridor in Virginia."

The cheapest addition to the plan would call for spending $20 million to link the Loudoun Parkway with the proposed Tri-County Parkway near Dulles.

Notably absent from the list of additions is the "Techway," a highway idea endorsed by leaders of America Online Inc. and other technology companies as a way to link employment centers in the Reston and Rockville areas.

Staff writer Lisa Rein contributed to this report.