Adding to the congestion on one of Northern Virginia's busiest highways, work is beginning on a new phase of the project to extend permanent, reversible rush-hour car pool lanes on the Shirley Highway outside the Capital Beltway.

For thousands of drivers traveling between Springfield and Quantico each day, the bad news is that work on this next phase of Northern Virginia's largest highway construction project will begin where traffic already is choked: the Franconia and Newington interchanges and the ramps leading to and from the Beltway.

Detours, traffic shifts and lane closings are likely along a stretch of highway that in places carries an average of 208,300 vehicles a day -- one of the region's highest counts.

"This is a massive, incredibly complicated project," said Mary Anne Reynolds, spokeswoman for the Virginia Department of Transportation, noting the combination of heavy traffic and utility lines along the project route. "It's the kind of thing the engineers will tell their grandchildren about.

"The project begins where the bottlenecks are, so it's going to cause more bottlenecks for a while."

"A while" is about three years. The $329 million project to extend car pool lanes 19.3 miles between Springfield and Quantico Creek in Prince William County is being done in nine phases -- up from the original five phases -- ending in 1996.

The first phase, which is just south of Franconia Road (Route 644), began in 1989 and is expected to be completed in June 1992. The next phase, which is just starting, involves a five-mile stretch from south of Edsall Road to north of Accotink Creek in Fairfax. It is supposed to be done in 1994.

When all phases are completed, car pools and buses will be able to use about 30 miles of high-occupancy vehicle lanes from the 14th Street Bridge in the District to just south of the Fairfax-Prince William line. Currently, drivers travel on temporary car pool lanes between the Occoquan River and Springfield and pick up the permanent, reversible lanes inside the Beltway. The permanent lanes are separated from regular traffic by barriers.

Although the project has been delayed beyond the initial timetable, Virginia transportation officials say they are giving it a high priority. The Shirley Highway car pool lanes carry three times as many people as the regular lanes. Every morning, for example, the two permanent car pool lanes between Springfield and Washington carry 39,000 people, while the four regular lanes carry 29,000.

The project not only is the cornerstone of Virginia's strategy to get more people to share rides to work, but will improve the traffic flow along the entire Shirley corridor when finished, Reynolds said. The work that is being done at the Beltway ramps and Franconia Road ramps also will help drivers using the regular lanes, she said.

In the next few months, crews will put concrete barriers along the shoulder between Edsall and the Accotink Creek to create a work zone for the initial construction. During this period, drivers can expect some lanes to be closed occasionally between 9 a.m. and 3 p.m. Most of the long lane closings will occur after rush hour, and drivers will be warned on signs of major closings several weeks in advance, officials said.

To build the car pool lanes between Edsall and the Accotink Creek, workers have to make several adjustments. Those include:

Shifting the northbound and southbound lanes anywhere from one to four lanes to provide room for the HOV roadway down the middle.

Widening the interchange bridges and ramps at the Beltway and Franconia Road.

Rebuilding the Newington interchange.

Building flyover ramps on the northbound and southbound lanes of the Shirley Highway that carry traffic from the car pool lanes to the regular lanes.