Drive southeast on Massachusetts Avenue toward Union Station, hang a right on New Jersey and, five blocks down, it should be a smooth left onto Constitution. But suddenly at Louisiana, there are those concrete barriers, again. And parked cars where an open street should be. And officers in shorts.

Instead, go out of your way and turn right, then left on First Street NW. But there's another police officer, looking in your back seat to make sure you're not concealing explosives or terrorists. It's not a huge imposition, but life in the nation's capital is different.

This month's street and sidewalk closures were the result of heightened security alerts, but they were hardly the first. In fact, almost from the moment the Capitol was built, the powers that be have found reasons to close roads in the name of progress, development and the national interest.

Washington, as planned by Pierre L'Enfant in 1791, has lost about 22 miles of streets over the past two centuries, most disappearing in a slow creep of small changes, block by block, over 100 years, according to local architect Donald Hawkins.

Although most of the lost miles were long gone with the advent of freeways in the 1960s, a recent rash of security-related restrictions since the 1995 Oklahoma City bombing and Sept. 11 terrorist attacks has commandeered more streets than in the previous two decades, raising tempers and questions about public safety.

A historical view serves as a reminder that access doesn't really vanish overnight but disappears gradually, slowly erasing a city's flexibility and choice. Cities evolve, of course, but modern security concerns have prompted more teeth-gnashing than usual: There's an emotional cost as citizens are repeatedly told what to do and where to go.

Closing the steps and terraces on the west side of the Capitol, for example, "is like closing the rim of the Grand Canyon," said Ken Jarboe, the advisory neighborhood commissioner for the area. "It's one of the most awe-inspiring views in the country."

The latest losses are the result of new dictates on where it is safe to drive and park and walk as Washingtonians and visitors navigate the symbols of power that make this city a likely target. New roadblocks and checkpoints have gone up around the Capitol, diverting pedestrians as well as motorists. Traffic is choked along 17th Street near the White House, where Pennsylvania Avenue is closed. And another block has closed just north of the World Bank on H Street.

"Oklahoma City kind of started the ball rolling at a bigger level, because you took Pennsylvania Avenue out and that kind of crossed the Rubicon," said Dan Tangherlini, the District's director of transportation, who gets daily requests to seal off streets or block lanes every time there's a new terror alert. "Then September 11th opened the floodgates. The streets that are closed don't even reflect a portion of the desire for people to close streets or take lanes."

Many people seem resigned to the increased scrutiny. But longtime residents argue that the average citizen isn't equipped to sensibly evaluate such threats. And as risk-wary experts push for maximum protection, those who live with the results say the small sacrifices of public access here and there have changed not only the way Washington works, but also the pace and feel of people's lives.

"It really is a shame to see our Capitol roped off. If security measures are really necessary, are only the top leaders being protected or is the public at large really safer?" asked Joe Baghetti, a graduate student visiting last week from Johnson City, Tenn.

"Cities are about options and variety," said Hawkins, who has studied the L'Enfant design for 30 years. "If you don't have the combination of streets and pedestrian areas, the streets lose life."

Hawkins heads the historic preservation subcommittee of the Committee of 100 on the Federal City, a planning advocacy group. He layered maps over the original plan to come up with his 22-mile estimate of lost streets, not including the more recent closings around Capitol Hill. The culprit over time, he said, is a combination of railroad tracks, highways, federal office construction and decades-old security concerns.

"The hugest portion of this mileage is one little block here and another block there," Hawkins said. "There are 100-plus different closures. It's a lot of little things."

About nine or ten miles have actually been added to the original plan. Riggs and Corcoran streets and Sunderland Place near Dupont Circle, for example. Streets near the National Portrait Gallery have disappeared and reappeared, closed for pedestrian malls but later reopened to traffic.

Not all street closures leave dead neighborhoods in their wake. Union Station dug up roads but invited progress, redevelopment and millions of visitors and commuters.

But scores of those visitors and commuters must now navigate checkpoints and concrete barriers as they visit the Capitol grounds, whose gently curving roads serve as parking for Hill employees. Pedestrians slip past barriers that have been in place since the early '90s to get to several of the paths. Watching over the scene are police cradling automatic weapons.

In Foggy Bottom, security patrols spend all day reversing their Jeeps and vans a few feet to let State Department employees into sections of C and D streets that are closed to the public. Next door, a block of street parking in front of the Federal Reserve has been sacrificed for a security buffer -- the city is collecting an annual fee as a tradeoff.

Construction continues just north of the White House, where Pennsylvania Avenue will become a multimillion-dollar pedestrian park before the next inauguration.

"We've lost access to the symbols of freedom, and in some cases we've changed the symbols of freedom to symbols of fear," Hawkins said.

Larry Molumby, who can see the Capitol dome from his home on East Capitol Street, remembers his daughter sliding down the west front of the Capitol on a sled in the mid-'70s.

"Maybe that's part of the problem. People wouldn't even think of recreating there now. It used to be like a neighborhood park," said Molumby, a retiree who still walks the grounds with his wife, Patricia, nearly every evening but notes that the military band concerts in the park now have more restricted seating. "It's just not the same."

Calculating the permanent loss of streets and right of ways in the capital is hard to do. For one, there are definitions to tangle with. Streets closed to cars but open to pedestrians are not always "closed" but merely "restricted."

Then there is the problem of memory. People feel strongly about the closure of Pennsylvania Avenue and E Street near the White House. They forget that West Executive Avenue between the Old Executive Office Building and the White House was closed during World War I.

By World War II, East Executive Avenue between the White House and the Treasury Department was shut. It reopened and then closed again after the 1983 bombing of Marine barracks in Beirut.

The U.S. Secret Service does not invite a discussion of alternatives.

"These measures are in place because we take our mandated responsibility seriously in providing the president and the occupants of the White House a safe and secure environment," spokeswoman Lorie Lewis said.

Street closures are also a muddle complicated by competing city and federal agencies that don't always work together.

District officials, who have jurisdiction over all streets except those on the Capitol grounds, say that only Congress and the D.C. Council can close roads. In practice, the Secret Service, the State Department and Capitol Police have all acted without first discussing alternatives with the city.

"Consultation doesn't mean they've sought our approval. They've just told us what they're going to do," Tangherlini said. "It still leaves the problem in my lap."

The problem with closing streets one by one is the lack of a broader review that takes into account citywide evacuation routes and other congested spots, Tangherlini said. D.C. officials sometimes describe the scope of the problem by naming roads not yet closed.

The Energy Department, for example, wanted to block off 10th Street just south of Independence because part of the street runs under a department building.

"What happens with the Labor Department -- do they then want to close Third Street because it runs under their building?" Tangherlini asked. "Trust me, they did."

Over on Independence and Constitution avenues, hydraulic metal plates are embedded in the ground, ready to cut off traffic on both major east-west arteries at the push of a button during the next emergency, Tangherlini said.

While city officials and community leaders say they wish that he public would ask more questions about losing access -- Are we really safer? What's the price in freedom and mobility? -- there are those who shrug at the latest inconvenience.

"I would have liked to walk up the Capitol steps, but they say you can't," said Sandie Byer, a retired school secretary from Sterling who sat listening to a fountain at the base of the west front of the Capitol this week. "I can accept that because of the terrorist threat."

Her husband, Frank Byer, retired from the Navy Department, conceded that it was a tough balancing act.

"You have six million people living in the greater metropolitan area, and they have a different need than the people who visit from Iowa and Illinois and across the country. I can come in or not come in. But this is a problem your generation is going to have to solve -- how to live this way and still be free."

"If you don't have the combination of streets and pedestrian areas, the streets lose life," architect Donald Hawkins said.Railroad tracks, highways, federal office construction and decades-old security concerns have changed the face of the city, Donald Hawkins said.