The year 2000 will be a groundbreaking one as transportation agencies across the Washington region plan work on dozens of road and transit projects, from huge highway interchanges in Fairfax, Prince George's and Montgomery counties to a new Metrorail station in Northwest Washington.

The most expensive item in either Virginia or Maryland is a $19.6 million project in Fairfax County to convert the intersection of Routes 28 and 29 near Centreville into an interchange. Scheduled to begin in the summer and continue through the fall of 2002, this construction is designed to keep pace with the rapid growth in the western part of the county as well as in nearby Loudoun and Prince William counties.

"This is a major bottleneck and headache for commuters during the morning and evening rush hours. By building a partial cloverleaf, we're going to be able to ease the congestion considerably," said Joan Morris, spokeswoman for the Virginia Department of Transportation.

Money for the construction was included in an initiative by Gov. James S. Gilmore III to accelerate the receipt of federal funds promised for the state.

This spring, the department also plans to start widening three miles of Telegraph Road between Beulah Street and the Fairfax County Parkway and between the Franconia-Springfield Parkway and Route 1.

Among the first projects to begin this year in Virginia is work to improve the Prince William County interchange of Interstate 95 and Route 234 and widen Route 234. Other major jobs slated for Prince William are the replacement of I-95 bridges over Route 619, improvements to the Route 28 intersections at Linton Hall and Vint Hill roads and rebuilding of the intersection of Routes 29 and 234 to add left-hand turn lanes and to make safety changes.

In Loudoun, VDOT intends to add two lanes to Route 7 between Dranesville and Lakeland roads and pave three miles of Woodburn Road, which is now a gravel byway.

Much of VDOT's attention over the coming year, of course, will be devoted to the ongoing reconstruction of the Springfield interchange, which began last spring. Replacing the interchange, at the junction of Interstates 95, 395 and 495, is currently the most ambitious, complicated and expensive road project in the Washington region.

A $40 million effort to complete the northern section of the Fairfax County Parkway is also continuing, with construction on the final 1.4-mile segment between Baron Cameron Avenue and Sunset Hills Road expected to start early this year.

In Maryland, the heftiest price, $17.3 million, will be paid to rebuild and redeck four aging bridges along the Capital Beltway in the Silver Spring area between Colesville Road and Georgia Avenue. This work, scheduled to last two years, is expected to severely affect traffic, adding to some of the worst congestion on the Beltway.

Another big-ticket item in Montgomery County is the construction of a new interchange on the eastern spur of Interstate 270 at the Rockledge Drive connector and an upgrade of the interstate's junction with Route 187. This effort, projected to begin this spring and to cost $15.4 million, is designed both to smooth traffic flow and offer better access to surrounding businesses.

The coming year will also see the state move ahead with the top county transportation priority in Prince George's: a new Beltway interchange at Ritchie Marlboro Road. The project, costing $15.1 million, is expected to ease congestion at the Beltway interchanges with Routes 214 and 4, officials said.

"It is a project which will provide great benefits to an area surrounding the Capital Beltway, providing safety and traffic flow improvements, and it's also going to help stimulate the economic development of the area," said Suzanne Bond, spokeswoman for the State Highway Administration.

Maryland also is scheduled to begin repaving and making safety improvements on the Beltway between Branch Avenue and Darcy Road in Prince George's.

Metro, meantime, is planning a series of transit projects, most notably construction of a new $60 million rail station near the intersection of New York and Florida avenues in Washington, if funding becomes available this year.

The Red Line station, located between the stops at Rhode Island Avenue and Union Station, would be part of the city's ongoing efforts to redevelop the New York Avenue area. Major property owners in the area previously have pledged to underwrite $25 million of the cost.

Also scheduled to start this year is the second phase of construction to expand the Metrorail station at Mount Vernon Square/UDC in preparation for the crowds expected to accompany the opening of the Washington Convention Center. This work, due to begin in February and to finish in the spring of 2002, involves enlarging the station's entrance and mezzanine.

Other new transit projects include building a pedestrian footbridge at the Prince George's Plaza Metrorail station, constructing canopies over station escalators now exposed to the weather and renovating the bus garage at South Capitol and M Street SE.