Washington area travelers may give thanks this week if recent improvements at Dulles International Airport succeed in preventing a replay of last year's holiday parking and traffic nightmare.

Last Thanksgiving and Christmas, airport officials had to broadcast radio warnings that all of Dulles' 9,200 parking spaces were full and apologized for exit-booth traffic jams that meant a 30-minute wait to leave the airport.

Since then, airport officials have added parking spaces, exit booths and valet parking at Dulles. Officials are studying plans for building additional roads and parking structures at Dulles and National airports.

"We should not repeat the problems of last Thanksgiving," said James A. Wilding, general manager of the Metropolitan Washington Airports Authority, which operates the two airports. "We've got more parking. We do not anticipate an overflow."

Although projections were not available, the airports are bracing for larger crowds than last year. The numbers of passengers using the two airports are climbing steadily, up 7.4 percent at National and 39.7 percent at Dulles in the 12 months that ended Sept. 30, compared with the previous 12 months.

Terminal crowds and ticket counter lines at National were starting to thicken Friday morning, said David A. Hess, spokesman for the airports authority. "It's already started," he said.

Officials at National, Dulles and Baltimore-Washington International Airport said last week that they are ready for this year's holiday crush, and they offered suggestions for minimizing travel hassles.

"We don't expect a parking problem" at BWI, said Linda Greene, spokeswoman for the Maryland State Aviation Administration, which runs the airport.

BWI, which has not had parking shortages, added 1,000 spaces to its satellite lots last week, bringing its total to more than 10,600 spaces.

Since last year, Dulles has added 2,100 parking space to its satellite lots and 373 spaces in the valet storage lot, bringing its total to 11,739 spaces.

Three additional exit booths, for a total of nine, should improve traffic flow, Hess said.

The valet service, launched in August, allows travelers the option of dropping off their cars in front of the main terminal.

BWI has long offered a valet service.

At National, the holiday headaches are attributable more to roadway congestion than to parking problems.

National's single-level roadway, an anachronism for a major airport, was adequate when the facility opened in 1941. Since then, the roadway has been choked with traffic.

The holidays increase the burden by flooding National with motorists picking up and dropping off passengers, Wilding said.

Some of those motorists may visit the airport only once or twice a year, so the roadway may confuse them, he added.

National usually has enough parking for the holiday crowd, but "we overflow with some regularity" at other times because of heavy business demand, he said.

Officials urged travelers intending to use any of the three airports to avoid parking and traffic problems by taking public transportation. {See box.}

Motorists are advised to avoid delays and traffic congestion by dropping off passengers in satellite lots, where shuttles will take them to the terminals.

The airports authority was created last year as a mechanism to finance about $1 billion in new terminals, runways, taxiways, baggage areas, roadways and parking at National and Dulles over the next seven years.

The authority's plan for Dulles calls for adding about 3,000 parking spaces, including a parking structure with at least one deck atop the lot in front of the concrete-and-glass main terminal.

The staff and board are debating whether one deck will be enough and whether more than one would interfere with the visual impact of the building, designed by Finnish architect Eero Saarinen.

"I would rather not have any structured parking in front of Saarinen's terminal," said authority board member T. Eugene Smith, calling it "probably the greatest piece of public architecture put in place in this country in my lifetime."

Smith said that one added level of parking might be possible but that "it has to be very, very carefully studied."

Board member William G. Thomas advocates building two or three decks of parking in front of the terminal. "I'm more concerned about passenger convenience than the ability to have a perfect view of the building from every place you can conceivably be," he said.

"I'm very sensitive to the concerns expressed about blocking the view of the building," Thomas said. "On the other hand, I'm concerned that we build as much parking at the closest location to the terminal as we can. We need to find balance between the two views."

Maryland's State Aviation Administration incorporated similar aesthetic considerations into its plans to build a parking garage in front of BWI's steel-and-glass terminal, said Jay Hierholzer, associate administrator of marketing and development.

The garage, which will add 6,700 spaces, will include four levels, with 1 1/2 levels below ground so as "to not spoil the visual impact," Hierholzer said.

The airports authority has not adopted a plan for National, but early proposals call for adding more than 3,000 spaces, bringing the total to 8,000.

The staff and board are studying a proposal to add most of the new spaces by building three parking garages on Parking Lot 2, just west of the Metrorail station, Wilding said. The structures would include three or four decks, with half a level below ground, he said.

The authority is considering including the first garage, which would add up to 1,400 spaces, among the construction projects to begin next year, he said.