After a decade of effort by the Pennsylvania Avenue Development Corp., it is no longer necessary to apologize for the route of presidential inaugural parades.

So decrepit that President John F. Kennedy was grossed out during his inaugural ride a quarter-century ago, Pennsylvania Avenue today is beginning to live up to its billing as The Avenue of The Presidents, America's Main Street.

A $75 million expenditure of federal funds for public improvements has spurred more than 10 times that much private investment in the blocks between 4th and 14th streets NW.

From the new John Marshall Park to the scaffold-shrouded Willard Hotel, from the U.S. District Court to the District Building, the signs of Pennsylvania Avenue's success can be seen on every block. Old buildings are coming down and new ones going up at a pace that should complete the 20-year reconstruction of the avenue on schedule in 1992.

By next Inauguration Day -- in 1989 -- the offices now being built will allow a new generation of workers to watch the parade; construction crews will be at work on the handful of remaining properties and the face of Pennsylvania Avenue will be determined for decades to come.

By then it will be too late to do anything about the one malignant failure that mars the street's stunning successes and haunts Pennsylvania Avenue planners like the ghost of inaugurations past.

The public-private partnership responsible for the Pennsylvania Avenue redevelopment has succeeded in every way but one: It has failed to create the new housing that is universally regarded as essential to the vitality of any city.

The failure to get more new housing built downtown is a failure not only of the PADC, but also of the District government, which has had no great success stimulating housing construction in the blocks adjacent to the avenue. Though the city has long been committed to "a living downtown" and has repeatedly called for making downtown "a people place," its accomplishments do not match its slogans.

Housing is at the top of the list of things that still need to be done on Pennsylvania Avenue, says PADC Director M. Jay Brodie. Brodie sometimes speaks of the Avenue's needs in abstract terms such as "animation" (of the streets), "identity" (as a neighborhood), and "linkage" (between The Mall and old downtown), but there is nothing abstract about the emphasis he puts on housing. When people live in a neighborhoood -- rather than just work or play there -- the quality of life changes for both residents and visitors.

So far, no one in Washington has found the formula for encouraging housing construction downtown.

We have succeeded -- to the point of saturation -- in fertilizing the market for office and commercial building, using most every technique of intensive cultivation to grow the rows of new buildings that stretch from Georgetown to Union Station.

In the last few years, Washington has anesthetized the myth that large-scale urban redevelopment will not work. Both Pennsylvania Avenue and the Metro Center project of the District's Redevelopment Land Agency are proof that government-planned mega-developments can build neighborhoods as well as destroy them.

But despite these successes in managing the market for office and commercial development, no one seems willing to challenge the judgment that the market will not support residential construction downtown.

The original PADC plans called for building 5,000 apartments along the avenue, most of them east of the FBI building. Because land downtown is deemed too expensive for residential construction, that goal has been scaled back to 1,200 units, or maybe 1,500 under the most optimistic scenario.

The first 425 apartments will be in two projects just getting under way -- the Westminster Development buildings at Sixth and Pennsylvania and Market Square between Seventh and Ninth streets.

These will be small, expensive apartments demanding Watergate rates for the right to share a Pennsylvania Avenue address with the President and First Lady. But even millionaires have to buy their groceries and their toothpaste somewhere. These first few hundred residents will stimulate development of real neighborhood shops and stores rather than the travel agencies, boutiques and money machines that predominate in office areas.

Unfortunately, that nucleus of live-in Washingtonians will have to grow by another 5,000 people to provide the critical mass necessary for a "living downtown."

Right now there are no plans on any drawing board for building that much downtown housing and no financing schemes around to pay for it.

Many of the forms of financial assistance that might be marshalled to get more housing are threatened by tax reformers and budget cutters. Federal housing subsidies, tax credits for perserving historic buildings, fast write-offs of real estate investments and industrial revenue bonds all could be used as creative financing for more housing, but none of them may be around long enough to do downtown any good.

Worse yet, the window of opportunity for creating housing downtown is beginning to close. As the office explosion continues, land is being used up and land prices are being forced up. By next Inauguration Day, it may be too late to launch any significant new housing projects.

What is so frustrating about all this, is that we know what has to be done, we just don't know how to do it. We face the prospect that Pennsylvania Avenue will be no more than a billion-dollar facade shielding America's Main Street from a dull downtown overlaid with block after block of boring office buildings.

At a time when the White House talks of less government and the District Building demands home rule, it is unfashionable -- if not impolitic -- to suggest that the federal government and the city administration should attack this problem together. But don't forget that everything that's been accomplished on Pennsylvania Avenue is the result of a presidential initiative that has been reindorsed by five successive administrations.

So far Pennsylvania Avenue's architects have kept their promise to revitalize The Avenue of The Presidents. Now perhaps the president can help us find a way to keep the promise of Pennsylvania Avenue.