Imagine you are driving into Washington at dusk, crossing the Roosevelt Bridge. To your right you glimpse the familiar image of the Lincoln Memorial, beginning to glow under the darkening sky. Directly in front hover the luminous, wing-like roofs of the U.S. Institute of Peace, a new landmark on the Washington horizon.

Or, say you are coming into the city from the opposite direction, following New York Avenue NE over the big, hump-like bridge above the railroad tracks. At the edge of what used to be a nondescript intersection at the bottom of the hill, where Florida Avenue crosses New York, stands a dramatic, aqueduct-like wall in a sweeping curve. Behind this, in the form of a sleek counter-curve, rises a new federal office building in a grid of stone and glass.

Architects and planners often talk about "gateway" structures such as these -- buildings that memorably signify entry or departure from a city or town -- but opportunities to actually design and build them rarely come along.

Think, then, how architect Moshe Safdie must be feeling these days after having hit the gateway lottery twice -- and in the same city, at more or less the same time. Safdie, the Israeli-born, Canadian-trained architect who burst on the scene in 1967 with his Habitat housing for Expo '67 -- the Montreal world's fair -- won separate competitions a couple of years ago to design both of these buildings for the nation's capital.

And on Thursday the Commission of Fine Arts enthusiastically endorsed his concept design for the Institute of Peace, a little-known federal entity, and his final plans for the federal building to house the Bureau of Alcohol, Tobacco & Firearms.

The bureau building is on track -- construction will begin in the spring, Safdie says, with completion scheduled for 2005. It will take longer for the Peace Institute's luminescent glass wings to rise above a corner of 23rd Street and Constitution Avenue NW. Congress pays the institute's operating expenses, but the organization must seek private funds to cover the building's $55 million cost. Institute officials say the building could be done by 2007.

Each building design is exemplary in its way, and the city will be lucky to have them both.

But approval of the institute design must come with a cautionary note. As splendid as it is, the design is somewhat bloated. The building is too massive for its tight corner site, and its exhilarating rooftop peaks maybe a bit too high above the trees.

Extreme care must be taken to make sure the new building does not compete with the Lincoln Memorial. The architect and his client and their federal overseers will have to get out their measuring sticks (or their computer simulators) to ascertain the proper balance between the two. The goal of such exercises is clear: The memorial is, and must remain, the preeminent gateway image in this part of town.

All the same, there's no need to meddle much with Safdie's design. An architect of his stature will be able to adjust it and yet preserve its essential qualities. Some shrinkage alone might do the trick. "Put it on a Xerox machine and reduce it to 90 percent," suggested architect David Childs, a member of the Commission of Fine Arts. Whether a reduction of 10 percent is sufficient remains to be seen, but this is a jewel of a design, and the proportions should be jewel-like.

Known only to a coterie of foreign-policy experts, the Peace Institute dates to a post-Vietnam War proposal to create a Peace Academy as a counterbalance to the academies for military officers. That idea got nowhere, says institute President Richard H. Solomon, but in the mid-1980s liberals and conservatives in Congress came together to support a reduced version -- "an independent, bipartisan institute committed to the prevention, management and peaceful resolution of international conflicts."

Clearly, when the Safdie design is built, the institute will be invisible no longer. Not only is its location close to the Lincoln, Vietnam Veterans and Korean War memorials -- Solomon quips it will become the "war and peace corner" -- but it also is the endpoint for a line of significant works of architecture on the north side of Constitution Avenue.

Aside from being too big, Safdie's design is a sure-handed response to these important contexts. His building does not reflect the symmetries and classical styles of its distinguished neighbors, but it respects them in cornice height, coloration of limestone-like concrete and the overall sobriety of its foursquare facades.

And then come the wonderful roofs. Formed with translucent white glass panels atop swooping lattices of steel, one glides toward the Roosevelt Bridge and the other sweeps out toward the Lincoln Memorial.

Corresponding to two cone-shaped interior atria -- a "private" space for researchers and administrators and a public area for educational programs and displays -- these floating structures symbolically suggest humankind's desire for peace and the institute's high aspirations. Architecturally, they are the poetic touches that bring the whole to life.

Safdie was selected to design the ATF headquarters through the General Services Administration's admirable Design Excellence Program. For that building he also was given a prominent corner site, but here the team at Moshe Safdie and Associates faced an entirely different set of challenges. Its responses were at once dramatic and subtle.

Security was the biggest issue -- the bureau, a law enforcement agency under the aegis of the Department of the Treasury, required the kind of security precautions usually allotted only to U.S. embassies in the most hostile of foreign environments. Yet its highly visible new Washington location is a centerpiece of the city's efforts to transform a barren stretch of territory into a lively, productive urban hub.

The match between location and use was not, in other words, made in Heaven. Safdie faced the contradictions head-on.

Does the client demand that its main facades stand back 100 feet or more from the city streets? Fine, the architect said, let's put the building in the middle of the site, and then let's split it into three parts and see what else we can do. Are exterior barriers required? Good, answered Safdie, let's make a whopper of a barrier, 30 feet high and half as wide.

Do city planners want retail stores along the street? Excellent, said Safdie and his mates -- let's build an entirely separate building for the stores, and put it in exactly the right place. Does the gateway site require a dramatic gesture? Great, came the response -- the big barrier itself can serve that purpose.

The resulting design, with three mid-rise office slabs situated behind that huge wall and the low-rise retail stores, is an improbable achievement, at once fortress and civic monument. The three office buildings themselves are reasonably elegant -- especially the curving, mostly glass facade facing New York Avenue. They make a pleasing composition and, with a large skylighted atrium and a spacious private park, form a healthy working environment. There are lots of trees.

And then there is the big gesture -- the barrier. It is a beautiful thing, a sweeping perforated wall that provides protection and yet projects a certain dignity. The strong curve makes a pleasing contrast to buildings behind it, and dramatizes the constant movement of the heavy automobile traffic on New York and Florida avenues. And, in addition to providing visual breaks, the large, regular openings emphasize the wall's rhythmic sweep.

"Don't call it a wall," Safdie said. All right, I won't. The 64-year-old architect has earned the respect. Enjoying a self-assured creative maturity, he's given Washington a pair of very different, very fine designs.

The wing-like roofs in architect Moshe Safdie's design for the U.S. Institute of Peace will greet travelers entering the city on the Roosevelt Bridge.An interior view of Safdie's design for the Institute of Peace, looking toward the Lincoln Memorial with one of the wing-like roofs pointing the way.The architect's design for the ATF's headquarters -- combining security and elegance -- is an improbable achievement, at once fortress and civic monument.Safdie made the most of the rare chance to design two "gateway" structures in the same city.