Imagine Northern Virginia 21 years from now. That's what the region's politicians have just done, and it's not a pretty picture.

By the year 2020, they say, the region's road networks will be even more jammed with cars than they are now. Getting to work during rush hour will be nearly impossible. And try just scooting around town in the middle of the day--you'll wish you hadn't, they say.

Armed with that depressing scenario, the politicians set out to create a transportation wish list, a 2020 plan that includes everything that might be built during the next two decades to help alleviate the area's transportation problem. Their proposal, adopted last week, will now be presented to the public before a final vote in October.

The plan recommends, among other things:

* Extending Metrorail's Orange Line to Dulles International Airport along the Toll Road and to Centreville along Interstate 66, and extending the Blue Line to Lorton.

* Building a Metrorail loop on the Capital Beltway and widening the freeway to allow car-pool lanes on both the inner and outer loops.

* Widening Routes 1, 7 and 28 to six lanes where they are not already, and widening some parts of Route 28 to eight lanes. Also, widening Routes 29 and 50 to six lanes in the outer counties.

* Constructing 60 interchanges across the region.

All of that--plus the reconstruction of the Springfield Interchange and the Woodrow Wilson Bridge and myriad other projects--could cost as much as $30 billion over the next two decades, according to Fairfax County Supervisor Sharon S. Bulova (D-Braddock), a member of the executive committee for the Transportation Coordinating Council.

Although the source of that money is not identified, Bulova said the council's proposed 2020 plan should serve as a goal for the region to strive toward.

Residents of Arlington, Fairfax, Prince William and Loudoun counties, Alexandria and the towns and other cities in Northern Virginia will have an opportunity to hear about the plan and provide input between now and the end of September, officials said.

"We need to have a plan that has been adopted by the region so that people are comfortable looking for a source of funding to make it happen," Bulova said. "It's not going to make everything perfect, but it will prevent things from being in total gridlock."

Not everyone is so sure.

James Wamsley is the transportation chairman for the Virginia chapter of the Sierra Club. He said his organization supports parts of the plan--especially the idea of building Metro to Dulles and on the Beltway.

But Wamsley said the region will never solve its congestion problems by building more roads or even Metro stations because growth left unchecked will always outpace the ability to expand the transportation network.

"This shows that you can't build your way out of congestion," he said, noting that the plan assumes Northern Virginia's population will increase by 562,000 by 2020, and the number of jobs by 403,000 during the same period. With that kind of growth, "it doesn't do the job."

The answer, say the Sierra Club and other slow-growth groups, is to limit population growth and to adopt more controls over where new homes and businesses are built.

"The problem is so intractable that you need to do everything," Wamsley said. Some members of the council--a planning organization made up of representatives of all the Northern Virginia jurisdictions--accept his point.

Arlington Board member Chris Zimmerman (D) said the group recognizes that its transportation plan won't solve all of the region's problems even if it does all get built by the time 2020 rolls around.

"I think it's a good starting point," he said. "There are questions that remain that have to be addressed in the next few years. We have major land-use challenges to face up to in our region. We've got to find ways to grow smarter."

Still, Zimmerman and others called the 2020 plan a much-needed blueprint.

Lois Walker (D), a member of the Alexandria City Council, said the document, once approved, will give Northern Virginia's politicians a better chance of securing funding for the projects, especially from the state's Commonwealth Transportation Board, which doles out transportation money.

"It forces us to make firm priorities and priorities that are not as parochial, but more regional," she said. "It gives Northern Virginia more of a united front when we go to Richmond."

Members of the regional council say they are hoping to avoid some of the more common criticisms during the upcoming public meetings because the plan strikes a balance between recommendations for bigger roads and more mass transit.

"We're very sensitive to potential criticism that we not adopt a plan that is more heavily weighted toward roads," Bulova said. "There's always a debate in this area between roads and mass transit. The answer is both. We need a bigger road network and more mass transit opportunity."

Wamsley said his organization will likely come out in support of some parts of the plan and in opposition to others.

"Beltway rail is a good project, and the Wilson Bridge is a bad project," he said. "We want to look at every project. There are good projects in there and questionable projects in here."