Several security and design specialists said yesterday that erecting barriers along the Mall to prevent incidents like this week's siege by a North Carolina farmer would change the character of Washington's monumental core so drastically that residents and political leaders would never accept the idea.

Various Washington landmarks, including the White House, the U.S. Capitol and the Washington Monument, have been ringed with Jersey barriers, bollards and other obstructions in recent years to protect them from bomb-carrying vehicles. But extending that approach to the Mall's vast perimeter is a non-starter, several experts said.

"You can prevent this from happening again [only] by making the Mall a very different place," said Steven Simon, an Arlington-based counterterrorism expert at Rand Corp., a nonprofit public policy institute.

"Right now, from a landscape and architectural perspective, what makes the Mall so appealing is the broad openness of the space. You'd have to construct significant barriers, berms or very stout fencing," Simon said. "Look at people how people viewed the closing of Pennsylvania Avenue [in front of the White House]. It was inhospitable and unsightly. The Mall is our village green. It's a space that we all share, where we all pursue our public and private pleasures."

On Monday, Dwight W. Watson drove his John Deere tractor over a curb on Constitution Avenue NW and into a shallow pond at Constitution Gardens, between the Washington Monument and the Vietnam Veterans Memorial, triggering a 48-hour standoff with police and street closings that snarled traffic throughout downtown Washington and on major commuter routes into the city.

But trying to protect the 146-acre Mall against other such incursions would be difficult and pointless, said former U.S. Park Police chief Robert Langston.

"Even if it was totally secured, people would just go somewhere else. They can stop right in the middle of the roadway in any downtown street, say they have a bomb, and have the same effect," he said. "I just don't think that in our society, we can prevent everyone from acting like a fool."

Reacting to complaints that many of the anti-terrorist barriers in downtown Washington are unsightly, federal and city planners in the past year have launched an effort to replace them with more visually appealing but equally secure protection. At the Washington Monument, for example, a federal panel has approved preliminary designs for a series of pathways and stone walls no more than 30 inches high to replace the Jersey barriers that surround the obelisk.

Charles H. Atherton, secretary of the U.S. Commission of Fine Arts, one of the panels that reviews proposed design changes on the Mall, said that "the essence of the Mall is freedom of access. And as soon you embark on creating such a space of freedom, you're bound to always invite the abuse of that freedom. I don't see how we can ever preserve that freedom and protect the park from the kind of the attack that it's been subjected to."