During the D.C. Preservation League's recent 25th anniversary celebration at the once-threatened Old Post Office Pavilion, the group cited eight of Washington's "most endangered properties." Among them were the L'Enfant plan and its network of streets and avenues.

Particular attention was focused on the closing a year ago of Pennsylvania Avenue on the north side of the White House. Nearly 400 people, dining together in the atrium, seemed to share a single sentiment: President Clinton should immediately reopen the avenue, restoring it as a functional and symbolic urban street, the street where the president lives.

Preservationists are not alone. Downtown property owners and office building occupants, retailers and restaurant operators, D.C. residents and commuters, tourists, planners, architects and members of Congress have joined the rising chorus of voices demanding the avenue's restoration. They all believe that closing Pennsylvania Avenue between 15th and 17th streets NW was unnecessary and inappropriate, as are most of the street-remodeling proposals advocated over the past 12 months.

This week, the National Park Service proposed reconfiguring the avenue into a curved street in front of the White House, but not reopening it. Trees, footpaths and fountains would be added in an attempt to evoke the character of the grounds during Thomas Jefferson's presidency.

Immediately after rifle shots pockmarked the White House on the last Saturday of October 1994, the Secret Service began seriously contemplating barricading the avenue. A week later, in a kind of preemptive but ultimately futile editorial strike, I wrote a column -- headlined "Blocking Off Pennsylvania Avenue Would Assault American Traditions" -- declaring the Secret Service's proposal "an incredibly bad idea!"

In opposing closure, I offered reasons echoed by protesters in recent weeks: disruption of traffic patterns, leading to more congestion downtown; adverse effects on nearby businesses; at best, marginal reduction of risk; and, perhaps most important, symbolic messages conveyed by unilaterally barricading the avenue -- "distancing the White House and its occupants from the American people" while disregarding entirely the views of Washingtonians and other agencies of government.

Unfortunately, in April 1995 the tragic bombing of the Murrah Federal Building in Oklahoma City traumatized the nation. Officials saw federal buildings throughout the United States as potential terrorist targets, and the White House already had been a target more than once. Returning the following month from South Africa, where I witnessed old barriers being removed, I was dismayed but not surprised to see new barriers being installed in America's capital city.

For 12 months, this two-block stretch of Pennsylvania Avenue has been open to pedestrians, joggers, cyclists and skateboarders, but not to vehicles.

The Park Service's proposal for converting the avenue from a city street to a landscaped, Lafayette Square adjunct may be an interesting exercise, but removing the barricades and reopening Pennsylvania Avenue to traffic should be the favored option.

Arguments for the restoration of Pennsylvania Avenue are based on sound transportation planning and urban design principles, reinforced by economic considerations and democratic ideals of openness and accessibility.

But to the Secret Service and other security specialists, such arguments probably seem romantic, naive and irrelevant to their mission. For them, a defensible security perimeter is the first priority, no matter what the collateral consequences might be.

Therefore, set aside for the moment "romantic" arguments and consider instead the validity of the security argument alone.

Is closing Pennsylvania Avenue to vehicular traffic really an effective strategy for protecting the White House? Does it significantly alter its vulnerability to dedicated or deranged terrorists? And how much security is realistic, for either the president or anyone else, before its costs and complications become extreme and unjustifiable?

Ultimately, there is only one reason for barricading Pennsylvania Avenue: to deter -- but not necessarily prevent -- the detonation of large quantities of explosives transported in automobiles, vans, buses or trucks parked on or passing by the north side of the White House.

I am no munitions or explosives expert, but this strategy seems fallacious on several grounds.

It cannot prevent a suicidal terrorist from reaching the fence along Pennsylvania Avenue with concealed weapons or powerful explosives. One can easily imagine such ordnance being transported on a person's body, or on several persons' bodies, as well as on or in motorcycles, bicycles and strollers. And really sophisticated terrorists probably could devise a system for propelling a small, explosive-laden vehicle over barriers from inside a truck parked just outside barricades or fences.

This strategy cannot prevent direct attacks from more distant positions with portable rocket launchers, rifles or even mortars, not to mention aerial assaults. With what certainty can White House antiaircraft defenses intercept a crazed but skillful pilot intent on dropping a bomb, firing a rocket or intentionally crashing a plane or helicopter into the White House?

Another fallacy lies in comparing the White House to the Murrah Building, which abutted a city street. The explosion in Oklahoma City was close enough to the building that the shock wave not only destroyed the building's facade but also demolished much of the building's structural columns, beams and slabs. This precipitated a chain-reaction collapse of the multi-story, concrete structure several seconds after the explosion.

By contrast, the north facade of the White House is set back 300 feet -- the length of a football field -- from the edge of Pennsylvania Avenue. At that distance, the building presents a much smaller profile. Further, its exterior walls are far more massive than were the Murrah Building's thin curtain walls.

Keeping traffic off Pennsylvania Avenue may lower the vulnerability of the White House to some extent, but is it enough to warrant doing such violence to the cityscape? Wouldn't it be equally effective simply to be more vigilant, to intensify surveillance of local traffic or, as architect Arthur Cotton Moore has suggested, to erect a transparent shield of impact-resistant glass behind the metal fence surrounding the White House grounds?

There are limits on how secure anyone living in a complex, technological society can be, including the president of the United States. And there are limits on how far we should go in pursuing security, especially when it may be marginal in its effect and carry a price too high to pay.

Of course, the Secret Service can contend undeniably that barricading Pennsylvania Avenue has worked so far -- after all, nothing has happened and, one hopes, nothing ever will.

Which reminds me of the guy who stood for hours on a street corner in the middle of Manhattan, continuously snapping his fingers. When a curious passerby asked why, the guy explained, "to scare away the elephants."

"But there aren't any elephants around here," observed the pedestrian. To which the guy replied, "See, it works!"

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.