For years, the Cabin John Bridge has been a pothole-strewn obstacle course for the 120,000 cars that use it daily to cross the Potomac River between Montgomery and Fairfax counties. The solution, highway planners decided, was to redeck the bridge.

Since last August, repairs have been making the situation even worse for Capital Beltway motorists, with two-mile traffic backups common on weekends. But when the work is finished in the spring of 1986, officials say, drivers' patience will be rewarded with a smooth roadway, wider shoulders, better entrance ramps and the possibility of more lanes in the future.

Repair crews have been closing two of the bridge's six lanes during off-peak hours. At night, the crews have been closing four of the six lanes at times, and one night this week they will close all three lanes of the southbound deck to Virginia for a 15-minute period between 11 p.m. and 2 a.m. Other, similar closings will be announced.

Last September, the local American Automobile Association attributed several rear-end collisions to the repair work on the 20-year-old bridge. But district engineer Mike Snyder of Maryland's transportation department said construction work did not cause the accidents.

"It's much better than we anticipated, frankly. We don't feel that it's the construction that causes the accidents," Snyder said.

The AAA has not monitored accidents there since September, but Tom Crosby, a spokesman for the group, said that as people become accustomed to the traffic backups, the accident problem diminishes. "I think it's people failing to pay attention to the warning signs," Crosby said.

Snyder and Crosby agree, however, that traffic congestion will increase as construction progresses and traffic lanes become narrower.

Potholes and "deck puncture" -- holes eaten all the way through the bridge so that the river is visible below -- have caused numerous backups on the span, officially named the American Legion Memorial Bridge.

The deterioration of the roadway, along with the accidents caused by funneling eight lanes of Beltway traffic into the six lanes of the bridge, were the main reasons that the AAA called the span the worst bridge in the area in 1983.

Road salt is considered the culprit that weakened the bridge's concrete and steel framework. Officials in Maryland, which inspects its bridges every two years, decided more than six years ago that the Cabin John span was in need of help, Snyder said.

Bridge reconstruction was put on the back burner until state and federal funds were raised from new gasoline taxes that started in 1981, Snyder said.

A new, denser type of concrete will be used for the new deck. The steel will be treated to prevent corrosion.

Snyder said the new deck should last about 30 years.

The repairs at Cabin John are part of a nationwide effort to fix deteriorating bridges. Studies showed that a shockingly large number of the nation's spans were in disrepair.

"Today we're in the throes of a major bridge-building program," said Stan Gordon of the Federal Highway Administration's bridge division. "The Surface Transportation Assistance Act of '82 provided $7.05 billion for fiscal years 1983 to 1986."

He added that other federal funds used for rehabilitating the interstate highways can be used for bridge repair, as is the case with the Cabin John Bridge.

The $14.3 million redecking of the bridge is more or less on schedule, as nighttime construction workers begin setting steel girders atop seven mammoth concrete piers that will support a new center section connecting the existing twin spans.

Redecking the bridge will involve removing the entire eight-inch-thick concrete roadway and its internal steel framework. The refurbished bridge will be wide enough to accommodate two additional lanes, if officials decide to add them later.

The Cabin John bridge is actually made up of three spans that cross the Potomac, the Chesapeake & Ohio Canal and the George Washington Memorial Parkway on the Maryland side, and MacArthur Boulevard. It is the section over the Potomac that is getting the full redecking.

The decks over the C&O Canal will not be replaced, but they will be joined by the new center section. Its concrete piers also will be reinforced, highway officials said. No major work will be done on the section over MacArthur Boulevard.

Plans call for the redecking to be finished by mid-December, according to project manager Mike Costa, whose Connecticut-based Lane Construction Corp. is redecking the bridge. Other work will extend the project until the following spring, with a deadline of June 1986.

Costa said he cannot recall working on a road project constrained by so much traffic. As an incentive, his firm will receive an additional $7,500 for each day it finishes the redecking before mid-December, up to 100 days. But if work falls behind, the company will be penalized the same amount for each day it takes beyond the deadline.

"Because of its strategic location, there aren't any other bridges that can be used effectively to allow those bridges to be taken out of service," Costa said.

"Everything humanly possible is being done to accommodate the free flow of traffic, but the sheer volume of traffic really complicates the project," said project engineer John Warnick of Maryland's transportation department, which is supervising the construction.

Three-fourths of the bridge, or 1,080 feet, belongs to Maryland, which will pay most of the repair costs, $11.5 million. Virginia, which owns 320 feet of the bridge, will pay $2.8 million for its share. The Federal Highway Administration ultimately foots 90 percent of the bill through the funds allocated to each state.

Traffic lanes will shift in three stages as sections of roadway are removed in strips that conform to the three existing lanes in each deck. Work on the northbound deck to Maryland will generally lag about a month behind the work on the southbound deck to Virginia.

Lanes will be shifted on the bridge beginning in the next few weeks and continuing through the end of the year as work progresses on the various lanes of the existing bridge and as the new center section joining the twin main spans is built.

Toward the end of the year, any or all of the exit ramps from the bridge that go east to Glen Echo and west to Carderock and Great Falls will be closed for brief periods between 9 p.m. and 5 a.m. Monday through Friday.