Tacky roadside architecture is an old if not always a venerated American tradition. Usually it is low-scale, packed together and dominated by parking lots and commercial signs. At its best it displays a quirky inventiveness and wired vitality. At its worst it's just an awful mess. And it can embody both qualities, very nearly at the same place and time.

Of more recent vintage -- dating back at least three decades -- are the attempts of "serious" architects to come to terms with higher-speed highways. The buildings they design are bigger and not at all dependent on signs for their identity. Indeed, whether they're abstract and modern or figurative and postmodern, these singular buildings can themselves be read as signs. One's eye registers them at 55 mph, and moves on.

Locally, one might characterize the commercial strip of Rockville Pike in lower Montgomery County as an example of the former idea, and the office-research corridor of Interstate 270, running a northwesterly route through the upper county, as an instance of the latter. The first is lined mostly with what Robert Venturi long ago defined as "decorated sheds"; the second has gathered its share of what he wittily described as "ducks," buildings to be seen and understood all at once.

The Miami firm of Arquitectonica, famous for its abstract flair, was asked several years ago by Ackerman & Co., an Atlanta-based developer, to create an eye-catching landmark for the new Washingtonian Center along I-270. The architects responded imaginatively by trying to bind these two traditions. Because of its bold, odd shape, the Rio retail building is an unforgettable, instantaneous visual event. But it's also decorated with signs, and it projects something of the tacky vitality of humbler commercial predecessors.

Arquitectonica is best known in Washington as the designer of the Center for Innovative Technology, the striking cluster of buildings perched on a highway-hugging knoll near Dulles Airport. At the Washingtonian Center (located just west of I-270 between the Shady Grove and Interstate 370/Sam Eig Highway exits) the firm is up to its by now tried-and-true tricks, confounding our expectations of rectangularity and notions of civility by designing the Rio building in the form of a trapezoid.

One end of the long building, supported by two canted black-painted pillars, angles out over a platform that juts into a curvy artificial lake. This is the high-identity eastern end, visible from the surrounding highways. Its upper face is like a minimalist abstract painting, an emphatic grid of white-painted steel and clear glass; tucked underneath this projecting facade is a transparent semi-cylinder, also a grid of steel and glass. A dashing, colorful sign -- "RIO" -- is pinned on a diagonal to the building's top right-hand corner.

This simple, high-energy facade is the building's most attractive and satisfying exterior element. Ironically, it is better appreciated from a footpath on the lake's opposite side than from the roads. From vantage points along the path one learns to respect Arquitectonica's ability to contrast plane and volume and to manipulate architectural forms as if they were huge pieces of minimal sculpture, set in ever-changing relationship to the natural and man-made environment.

Due to a westward rise in the ground the building's other end, though also angled outward, is much less prepossessing; despite its multicolored windows and a spiffy sign for the movie theaters inside, it's an auto drop-off, a secondary entrance. The two long sides of the building are warehouselike, totally plain Jane despite attempts to enliven them with graphics and a fire stairwell with a nifty trapezoidal screen.

In an apparent attempt to defer to Washington's circumspect architectural manners, the architects, urged to do so by the developer, opted for a bland, gray, synthetic sheathing rather than a vivacious aqua green, as originally planned. This was a major tactical miscalculation, an excess of caution -- decorating the shed was a big part of the game here. Instead, the building's utter ordinariness -- it is a warehouse, although the "goods" it stores are human activities -- is nakedly exposed.

This mistake aside, the Rio building is a terrific example of serious modern architecture producing a fun building for a lively, consumerist purpose: Though rentals have been slow since it opened a year or so ago, the structure was conceived as the retail centerpiece for the 212-acre Washingtonian Center mixed-use development. At present it houses a couple of lonely restaurants and shops, an eight-theater movie complex and one of the more architecturally appealing sports-and-health clubs this side of California.

Predictably, the interiors are first-rate, although the inexpensive materials occasionally employed (e.g., wallboard sides for the main stairwell) will cheapen the effect when and if crowds arrive at the Rio building. Straightforward floor plans are enlivened by large, podlike receptacles for stores in the center of the mall, but most of the spatial drama is concentrated, appropriately, at the eastern end. Here, in a high lobby enclosed by the semicircular window grid, Arquitectonica again demonstrates its skill at deploying diagonals in space: The bridge, stairwell and escalators make a splendidly dynamic composition. The space is a bit too tight, but even so, experiencing it is like being somehow privileged to walk inside a painting by Kandinsky in his geometric period.

The most exhilarating space, however, is the exercise room behind that bold, uppermost window grid: It's a soaring, light-filled chamber where men and woman can work the Nautilus equipment with tremendous views of the sky or, if they choose to labor away on the aerobics balcony, with vivid perspectives over the lake and, beyond that, over the roaring traffic of I-270. Throughout, Rio's sports facility is an energizing contrast to the dismal windowless basements that are the unfortunate metropolitan norm for such operations.

Arquitectonica's dedication to abstraction is consistent and perhaps a bit poignant -- even the plastic fire-warning labels outside the elevators are like little abstract paintings. The firm, headed by the husband-and-wife team of Bernardo Fort-Brescia and Laurinda Spear (and working here in collaboration with Ward/Hall Associates), makes a practice of trying to do a lot with a little -- sometimes too little, but more often just enough, as in the splashes of colored tile arranged just so, or the stairwells zipping this way and that.

It can be a cartoonish sort of architecture, to be sure, but it's done with sophistication and engaging grace, a certain cocky confidence that the 80-year-old tradition of abstraction can be and indeed is a popular art form. The firm has helped to reinvigorate modern architecture by turning '60s ponderousness on its head in a spirit of play. It's a refreshing, anti-stodgy posture that is especially appropriate to a building such as Rio, whose purpose is to celebrate nothing more ennobling than the act of shopping or of being entertained.

Those who would argue that the strategy of designing such singular, abstract buildings is inherently anti-contextual should carefully consider the specific context. This is roadside architecture, after all, and the Washingtonian Center is all new and, so far, all modern: Witness the adjacent, 14-story Bechtel building (designed by the Washington office of Hellmuth, Obata and Kassabaum), with its blue mirror-glass sheathing and its lumpy scalloped top.

No, the Rio building sits where it is quite comfortably, albeit with an undeniable eclat. This is not merely a question of style; even more important are the issues of land use and pattern. The Washingtonian Center is by no means an ideal example of a mixed-use settlement -- it's way too dependent on the automobile, and its 700 residential units (out of 1,400 planned) are adamantly segregated from the commercial core. But the site plan (also by HOK) is in many ways excellent, and the Rio building fits into it like a perfect puzzle piece.

One thing that's to be regretted, of course, is those not-green walls. Otherwise, welcome to Washington, Rio.