"900 Ninth Street? What's there?" the cab driver asked.

When told it was the convention center, he laughed. "That place," he said, "that place will never open."

Surprise. After more than a decade of talking and lobbying, and after four years of planning and construction following the first congressional appropriation in 1978, the $98.7 million Washington Convention Center will come to life within a month.

A great whale of a building that dominates a large portion of the District's deteriorating downtown, the center will be opened Friday with an official ribbon-cutting ceremony. Its first event -- the five-day National Capital Area Auto Show, expected to draw 6,000 people -- will begin Jan. 5.

It's nice gravy for the convention center managers to open with an event that large. The key to the future success of the project, however, comes in March, with a meeting of a little known but, in the world of conventions, extremely influential American Society of Association Executives.

George W. Demarest Jr., the convention center's general manager, called it "a convention of our potential."

The group's members are the ones who determine where the organizations of America hold their annual meetings, which can bring tens of thousands of free-spending visitors to a city -- the people the convention center was built to attract.

"Our success will be measured by the groups of 10,000, 20,000 or 30,000 [person] conventions that will spend in the Washington area an average of $500 a person during a four-day stay," said Demarest, a native New Yorker who is one of three outsiders brought here to run the center.

These groups, however, are sitting on the sidelines with tentative, but unlisted bookings, waiting to see how the center performs.

"They are there. They are very interested. They are just waiting to see if we can cut it," said Alan Grip, assistant general manager for marketing and public relations.

If all goes well, added Assistant General Manager Michael C. Rogers, "we'll be bursting at the seams" in 1986 and on, even if only half the tentative bookings come through.

Until then, though, "We are going to be tested, squeezed, felt and examined. We all know that. We are geared for that," said Demarest, who ran convention centers in Dayton, Ohio, and Cleveland before moving here.

Most of next year, then, is taken up with local shows open to the public and small trade shows that will help pay the center's operating expenses but not fulfill its basic goal -- attracting out-of-town conventioneers to the city.

Nonetheless, said Demarest, many of those shows could not be held in Washington without the convention center. For instance, a gift-item show, scheduled for mid-January, had outgrown Washington's available hotel ballroom facilities. Two shows put on by the World Computer Graphics Association likely would have been scheduled for Baltimore because of a lack of facilities here.

Caby Smith of the World Graphics Association said technology moves so quickly in his field that he could not have waited the three-to five-year period it would have taken to get suitable hotel ballroom accommodations in Washington. "The opportunity would have been gone in two years," the association executive said.

But these shows do not bring in people from elsewhere as do the large conventions, which the center was designed to accomodate. For instance, 48 events are booked into the convention center in 1983, 19 of them local public shows. According to figures compiled by Assistant General Manager Alan Grip, the 29 smaller conventions and trade shows will draw an estimated 144,550 people to the convention center and bring $72.2 million to the city.

There are a few big-number conventions scheduled for 1983 for the center's staff to cut their teeth on. The Christian Booksellers As could be the beginning of Washington's place on a triennial cycle of rotating sites. The Washington-based American Chemical Society will hold its August meeting here, with an anticipated attendance of 10,000, and a large printing equipment show expected to attract 20,000 participants is booked for October.

In 1984, when the larger conventions start rolling in, 23 events are expected to attract a greater number of people -- 240,000 -- and more money -- $120 million, according to Grip's estimates.

Five of the 1984 gatherings will use the full facilities of the convention center: a major construction show called "World of Concrete" (20,000 people expected); Baptist Fundamentalism (20,000); the American Booksellers Association (16,000); the National Education Association (12,000 to 14,000), and the Radiology Society of North America (20,000).

The radiologists are considered a big draw, and Demarest said they are looking to return here on a four-to-five-year rotation cycle. Furthermore, their convention is scheduled over Thanksgiving weekend, when most hotels in the city have an abundance of rooms.

When the real test comes, for bookings fall sharply in 1985 and stay down while the big convention schedulers watch to see if Washington's facility has proven itself.

Demarist feels there is little chance of failure. "As people come in and see the building," he said, "they see that it's a very functional, flexible convention building."

The centerpiece of the convention center is its giant second-floor central hall, which covers 150,000 square feet of space. It is a bare chamber with a concrete floor and construction girders showing in the ceiling, but it is so immense that four large tractor trailers standing in the middle of the floor look like toys.

The carpeted showcase hall is on one side of the large hall, covering a mere 26,000 square feet. It will be used for banquets and other fancy functions, including Mayor Marion Barry's forthcoming inauguration ceremony. As a lure to international meetings, it has six soundproof translation booths overlooking the floor.

A third large convention room, 100,000 square feet of floor with no beams to obstruct sight-lines, is on the other side of the large center hall.

The center's fourth large hall is on the street level, as are 37 smaller meeting rooms, a kitchen, cafeteria and a cocktail lounge, which will be open to the public, facing the corner of Ninth and H streets NW.

Unlike many recent construction projects, the building was completed on schedule and slightly under budget. Although work is still going on, the center will be swept clean and all signs of construction should have been removed by Friday afternoon's ribbon-cutting ceremony.

The center's reason for being was to lure big conventions with their out-of-town money to Washington, to fill hotel rooms, create new jobs and, as a bonus, act as a nucleus for the regeneration of a part of the city's old, dilapidated downtown area.

Center officials project that the facility will generate 3,500 new jobs by late 1986. Most will be in retail, transportation and service business and the hotel industry -- taxi drivers, front desk clerks, waiters and waitresses, what Demarest called "people industries."

Whether the convention center will keep its visitors downtown remains problematical, though some nearby businesses are working on promotions to attract conventioneers. Woodward and Lothrop, located 2 1/2 blocks away, for instance, is involved in promotions with next month's auto show.

"We generate bodies. A businessman has to go out and grab them," said Demarest.

The center was built with money from long-term U.S. Treasury loans, and the interest and operating expenses in the early years come from two special taxes: an 80 cents-a-night levy on occupied hotel rooms and half of the 10 percent surcharge on business franchise taxes.

But Demarest said the center has never been expected to be a moneymaker, though management is charged with reducing the deficit. The rational, however, is that the new money it generates for the city will far outweigh its operating costs.

And by 1987, center officials estimate, it will be responsible for added tax revenues that will exceed its operating expenses.

The center will make money directly from renting space and services such as utility connections for exhibitors. Rentals by September, Rogers said, will amount to $764,000. Furthermore, each exhibitor is expected to spend an average of $150 for electrical and telephone connections.

The center's 1982 budget, was $1.2 million. That jumped to $7.5 million in fiscal 1983, which started Oct. 1, because of large capital expenses, including the purchase of theater-style banked seats needed for general sessions of about a dozen conventions scheduled for the next few years.

This seating has come under attack in Congress from allies of Capital Centre owner Abe Pollin, who are trying in a D.C. spending bill to bar the convention center from booking circuses, sporting events and concerts, a major revenue source for the Cap Centre. That move is opposed by the city government, although convention center officials have insisted they have no intention of booking the kind of events the bill is aimed at. CAPTION: Picture 1, View from corner of Ninth and H Streets of Washington Convention Center, set to open this week after four years of planning and construction at a cost of $98.7 million. Photo by Joel Richardson -- The Washington Post; Picture 2, WASHINGTON CONVENTION CENTER, Chart bt Nathy Jungjohnanson for The Washington Post; Picture 3, Workman prepare flooring in convention center hall., Photo by Joel Richardson -- The Washington Post