The Cabin John Bridge, which links Montgomery and Fairfax counties via the Capital Beltway across the Potomac River, may lose top billing as the metropolitan area's major traffic headache when the bridge deck's replacement winds down during the next two months.

The final phase of redecking the bridge should begin in the next two weeks, with completion of the roadwork projected by Dec. 16. When the work is completed, the number of lanes will be increased from three to four in each direction and long-term damage to the bridge from road salt will have been repaired, according to Maryland highway officials.

The massive repair project, whose $14.3 million cost is being shared by Maryland and Virginia, has been under way since September 1984, causing long delays not only for rush-hour commuters but also for midday traffic, to which some lanes are closed.

The bridge, officially named the American Legion Memorial Bridge, carries about 120,000 vehicles a day.

The American Automobile Association, which in 1983 said the Cabin John Bridge was in the worst state of disrepair of any bridge in this area, has logged an average of 90 calls a day on its Cabin John Bridge hot line, 222-6200, which provides a tape recording of lane closings and work repairs.

Next week, the three divided lanes of traffic on the northbound deck will be rejoined so the outer lane and the bridge railing can be replaced. The divided southbound lanes were brought back together last week.

Two of the six lanes are closed between 9 a.m. and 3 p.m. on weekdays. The bridge repair contractor, Lane Construction Co. of Meriden, Conn., agreed with Maryland officials to keep three lanes in each direction open during the morning and evening rush hours and has done so, according to Douglas R. Rose, the bridge's project engineer from the Maryland State Highway Administration.

"We monitor the closings of the lane," Rose said. "One day it might have been closed a few minutes early. We get a lot of complaints if we close one minute early or open one minute late."

Last month, WTOP airborne traffic reporter Bob Marbourg reported that construction workers had closed off lanes before 9 a.m.

"It seemed like they were anxious to accelerate their schedule," Marbourg said in an interview. "They were in the right lane of traffic of the northbound deck, so they could block the lanes exactly at nine."

Marbourg said that he talked to highway officials and believes that no premature blocking of lanes has occurred since.

"We've gotten a handful of calls saying, 'Hey, the cones were out and lanes were closed before9 a.m,' " said AAA spokeswoman Mary Ann Reynolds.

On Saturdays the construction company can close lanes any time; on Sundays, when no work is done, all lanes are open, Rose said.

Maryland has agreed to pay the company a bonus of $7,500 for each day that the work is finished before Dec. 16, or to penalize it the same amount for each day that work continues beyond Dec. 16.

"It's an optimistic schedule, but we're shooting for that deadline," said state highway engineer Michael Snyder. "The biggest factor will be the weather."