It's 78 down and 948 more plastic-coated slabs to go before construction crews will finish the scheduled 14-month redecking of the Woodrow Wilson Bridge, a task that has diverted, slowed and sometimes interrupted the bridge's thousands of daily commuters.

But the contractor for the $24 million project says all is going well, and project engineers forecast the redecking will be completed several weeks before the promised date of Jan. 4, 1984.

That should be good news for the more than 100,000 motorists who daily ramble over the Wilson Bridge, a 21-year-old span whose badly crumbling roadway has concerned highway officials for years.

The reconstruction work has contributed to some major traffic backups, late deliveries to businesses on both sides of the bridge and, according to the American Automobile Association, has persuaded about 4,000 motorists to take alternative routes each night when redecking work shuts down four of the six lanes of the Interstate 95 link.

On the whole, however, the bridge work has received high marks, including praise from the AAA, which points to the difficulty of laying in 27-ton slabs during midwinter nights and the general success in keeping all lanes open during peak hours.

James Bryce, project engineer, says "we're well on schedule, but not ahead, yet. We're finding a lot of answers to a lot of unknowns we had in August when we planned this job."

Two of those unknowns, Bryce said, were how harsh the Washington winter would be and how difficult it would be to cut and lift out the bridge's old concrete decks. Since work began in mid-November, he said, both have turned out to be less troublesome than expected.

Starting Jan. 30, 10 to 15 additional workers will join the 90 or so currently on the project, which is being handled by Cianbro Corp. of Pittsfield, Maine. Project manager Richard Dooley said the new workers will start at the Virginia side of the bridge and work toward its midpoint as the present crew works from the bridge's midpoint toward the Maryland shore.

The project marks the first time a major bridge in this country has been redecked while remaining open to rush-hour traffic, Bryce said. Almost all redecking work is still done at night: Monday through Thursday from 8 p.m. to 6 a.m., and Friday from 9:30 p.m. to 7:30 a.m.

Soon, redecking will also be done on Saturdays from 8 p.m. to 10:30 a.m. to make up for any weekday work suspended because of bad weather.

There are traffic backups during the night work hours because only two lanes are open, but the really big snarls have occurred on the several occasions when work crews were unable to reopen the full bridge in time for morning rush hour, according to Thomas R. Crosby, spokesman for the AAA's Potomac Division.

Early last month, when workers ran into trouble fitting a temporary steel plate into the roadway, traffic backed up for more than 30 minutes, he said.

In mid-December, when Washington got hit with seven inches of a sneak snow storm, bridge crews were slow reopening the bridge. More backups.

And as recently as Tuesday, motorists were backed up for miles because of a bomb scare that caused the bridge to be closed.

But, Crosby said, "we at AAA feel they the contractors are doing an excellent job. If they achieve what they have set out to do, it would be an engineering feat."

To help minimize traffic jams, AAA has advised motorists to avoid the bridge during construction nights. As of last week, Crosby said, AAA had received more than 32,000 calls on its special hot line, and circulated more than 10,000 maps pointing out alternative routes to avoid the bridge.

Crosby said AAA has even had Capital Centre announcers remind its thousands of patrons to avoid the bridge when they leave for home. And, according to Crosby, the measures seem to be working.