I have seen the sidewalk of the future, and it is on Pennsylvania Avenue NW.

Specifically, it is the long concrete expanse in front of the sleek new International Monetary Fund building on Pennsylvania -- and also along the shorter sides of the same building on 19th and 20th streets. And along H Street, too, in the back of the block-filling edifice.

This is in many ways a pleasant sidewalk, with an attractive fountain and more benches per linear foot than most pathways in urban parks. It has young willow oak trees that will grow to handsome maturity, interspersed with those always appealing, double-crowned "Washington" street lamps.

But it is also strange. The sturdy-looking benches are really sturdy. They have been constructed to withstand the head-on impact of a car or truck laden with explosives. So, too, have the burnished steel cylindrical bollards that punctuate the spaces between the benches -- and the snazzy-looking bus stop shelter, with its sharply angled steel-and-glass canopy.

These sidewalks, you see, are the first to be constructed for the most part in accordance with guidelines established three years ago in the National Capital Planning Commission's "Urban Design and Security Plan." They are proof positive that, up to a point, you can combine safety with beauty in street design.

That, in short, was the point of the plan, which was initiated by a feisty, smart Boston developer named Richard Friedman when he was a commission member in search of a creative response to a pressing problem.

Washington was becoming uglier by the day with the assortment of barriers and concrete flower pots being thrown up around federal buildings and monuments. Friedman wanted to enlist the nation's design community in an effort to meet the capital's security needs while eliminating the spreading security mess.

The resulting plan established minimum design standards for a variety of conditions and Washington locations. There were no one-size-or-shape-fits-all prescriptions. The sidewalk guidelines, for instance, set maximum distances between elements and emphasized that lots of ordinary street furniture -- benches, trash cans, lampposts -- could be strengthened to withstand a significant impact.

Individual clients and designers were expected to rise to the challenge. The IMF and its distinguished architects, Pei Cobb Freed & Partners of New York, voluntarily sought the planning commission's advice on sidewalk fixtures, and the architects then went about their business.

They did well. By deploying just a few abstract shapes -- rectangle, circle and trapezoid -- and by using a harmonious palette of materials -- fine concrete, mottled granite panels, brushed steel, square Belgian block pavers -- they ensured that the IMF sidewalk would be read as a (mostly) appealing whole.

Ingeniously, the architects combined benches with that tired staple of curbside security, the heavy flower pot, to make the basic, repeating unit of their design. Measuring about 9 feet long by 4 feet wide by 31/2 feet high, this clean-lined unit looks very solid.

Yet it is unquestionably attractive. A wooden bench is set securely into the structure, most times on one side only. A trapezoidal basin filled with seasonal flowers (pansies, for now) makes an emphatic, yet gentle, end piece. On a breezy fall day, this is indeed a pleasant place to sit.

And then there is the fountain. The city insisted that some kind of water element be included in the design, and the architects (collaborating with Wet Design of California) came through brilliantly with an enticing water wall that forms a sweeping curve next to the main Pennsylvania Avenue entrance.

Despite all that, the question remains: How many of these formidable sidewalks do we really want to see? There is a certain contradiction between city life and these elaborate, permanent security precautions.

Even this handsome effort, for instance, creates a definite wall-like atmosphere. Walking along, you feel a bit penned in, an impression only strengthened by the elimination of curbside parking on all four sides of the building. It's not quite normal, not quite expected.

And that is on the Pennsylvania Avenue frontage. On the H Street side, very much treated as a back door, we get to see a prime example of bollard mania -- a long row of steel cylinders, spaced 36 inches apart, gleaming in the sun. It raises the thought of how bad things could get when designers of lesser skill are involved.

Furthermore, as with the original IMF headquarters building immediately to the south along H Street, the nearby World Bank complex and International Finance Corp., this new IMF building is sealed off from the public at large. You cannot get into the splendid atriums nor excellent cafes and food courts without an invitation or an escort, and a security screening is necessary before you can even buy a book at the institutional bookstores.

So, like many federal buildings these days -- think of the Federal Triangle array on the south side of Pennsylvania Avenue between Sixth and 15th streets NW -- neither the new IMF structure nor its sidewalk is a billboard for the lively street life we'd all like to see in our downtowns.

The system of massive if reasonably comfortable benches on the avenue seems to be a sensible precaution against vehicles packed with explosives, a weapon favored by international terrorists. Homegrown ones, too, as we saw a decade ago in Oklahoma City.

And yet, sitting on one of the "hardened" benches in the autumn sun, one has plenty of time to hope that they prove to be evidence of an aberrant present, rather than projections of a normal future. All in all, I'd take a plain old street bench any day.

Benches outside the new IMF building combine protection, aesthetics.Architects Pei Cobb Freed & Partners, working with Wet Design of California, designed a water wall that creates a sweeping curve next to the main Pennsylvania Avenue entrance of the new International Monetary Fund building.Street fixtures such as bus stop shelters are built to withstand a significant impact, yet remain aesthetically pleasing.Bollards on the H Street side of the building offer an unhappy contrast to the Pennsylvania Avenue side.