Regional officials planning how the Washington area would evacuate quickly in a terrorist attack say they have come to a sobering conclusion: It can't be done.

However, in all but the most remote scenarios, that won't be necessary, they say.

Noting that the region's road and transit network buckles under the pressure of a normal rush hour, government and transportation leaders are focusing less on a mass evacuation than on moving smaller numbers of people from local danger zones.

Emergency planners say they soon will mount a campaign telling people that, unless they are in the immediate vicinity of an attack, they should resist the instinct to flee or rush to their families. Instead, planners said, people will be asked to help prevent a frantic -- and unworkable -- exodus by waiting for instructions. For most people, planners said, the message probably would be, "Shelter in place."

A key challenge for planners is building public confidence in the instructions people would receive in an emergency.

"The first instinct should not be to pile into your car, because you're not going to go anywhere," said David Snyder, chairman of the emergency transportation work group of the Metropolitan Washington Council of Governments. "All you'd do is potentially harm yourself and jeopardize other people who need to respond to the incident or get out of the area."

COG, a regional coordinating agency, is working on much of the evacuation planning with counties and cities, the state and D.C. transportation departments and the federal Department of Homeland Security.

Federal and local officials haven't had to do such complex evacuation planning since the Cold War. Preparing for a terrorist attack is far different from preparing for a nuclear strike, they say.

Cold War planners assumed that an escalating world crisis would provide several days of lead time for orderly evacuations of entire metropolitan areas. Today, emergency planners say, they are preparing for smaller attacks that could come without warning and involve a mix of threats: biological, chemical or radioactive. They also contemplate multiple attacks and ploys to get people into the open, where they would be vulnerable.

The problem with a mass evacuation plan is that the region's roads and transit systems, already swamped on a typical weekday, can't handle a sudden surge.

The Capital Beltway, which slows to a crawl after one fender-bender, would come to a grinding halt if thousands of panicked people hit the road at once. Motorists and bus riders trapped in traffic could be exposed to an airborne threat or other types of attacks. The more that roads, trains and buses are packed with people fleeing, emergency planners said, the more difficult it would be to move in police, firefighters, emergency supplies and the military.

Local governments have distributed maps showing evacuation routes, and federal Homeland Security officials have advised families to keep emergency kits in their cars. But such preparations depend on public order.

"If an event creates widespread fear and panic, that's a tough scenario." said Richard A. White, Metro's chief executive officer. "We have a finite capacity. I have the sense that not enough people in the region understand that."

Dan Tangherlini, the District's transportation director, said computer studies estimate that, at best, it would take four to six hours to evacuate the city's workday population of nearly 1 million people. That assumes that no accidents, construction or illegally parked vehicles are blocking lanes. It also assumes that local officials can get people to act in the manner of the traffic models.

Still, Tangherlini said, worst-case scenarios are misleading. Federal intelligence and government models suggest that the most devastating threats are the hardest to carry out. The likeliest dangers affect smaller numbers of people.

"Too much effort, energy and focus has been put on the least likely scenario," Tangherlini said. "You're talking about the nuclear war scenario, which is not the terrorist threat scenario."

But some local transportation watchers question whether governments can overcome the human instinct to escape office buildings and hit the road to pick up children, get home or flee. Bob Chase of the Northern Virginia Transportation Alliance, an advocacy group, said he doubts that many people would agree to stay put if they felt their families were threatened.

"I think you'll have a situation where a lot of people will say, 'I know this is what being a good regional citizen is, but to hell with that. I'm going to Manassas,' " Chase said.

Chase said he questions why emergency planners haven't done more since Sept. 11, 2001, to address well-known traffic problems, such as the limited number of Potomac River crossings, that would stymie any evacuation.

The idea of asking people to "shelter in place" isn't one that Snyder expects will comfort an uneasy public fresh off a Code Orange duct-tape-buying frenzy. He worries that emergency planners won't know if they can pull off partial evacuations until something happens.

"We realize that people aren't fully comfortable with that yet," said Snyder, a Falls Church City Council member.

The federal government has begun trying to build public confidence. Last month, 80 chiefs of staff from the executive, legislative and judicial branches were briefed about phased evacuation plans for 350,000 federal workers.

Michael F. Byrne, recently named Homeland Security Department coordinator for the capital area, drafted the emergency protocol. Steps range from partially evacuating a single building, to a staggered evacuation and lockdown of surrounding areas, to a full shutdown like the one during the 2001 terrorist attacks.

"This is a complicated process, made more so as no one is legally impeded from self-evacuating if they so choose," Byrne said. "A staged release could quickly become a de facto full-scale evacuation."

Officials at the U.S. Capitol and some federal agencies are telling employees about emergency plans to lock down buildings and garages, and D.C. police are giving similar information to businesses. That would secure facilities and ventilation systems while also reducing traffic.

What it would take to persuade people to ignore their desire to flee is unknown, but it could be difficult, researchers in evacuation behavior said.

"Really, no research has been done on what kinds and what levels of education you need to get people to shelter [in place] versus evacuate," said John Sorensen, a researcher at Oak Ridge National Laboratory, who has studied the public's response to emergency warnings for the Federal Emergency Management Agency.

However, Sorensen said, studies have shown that people aren't likely to comply with government directives to stay put unless they are told specifically how it would make them safer.

Sorensen said a study of a toxic explosion at an Arkansas chemical plant in 1997 found that most of the nearby residents who were told to stay in their homes, rather than evacuate, fled anyway. The evacuees later said they ignored that advice because they didn't believe they were given good reasons to stay, Sorensen said.

Local officials say they have focused first on preventing the kind of confusion that happened Sept. 11, when workers hearing of the World Trade Center and Pentagon attacks flooded out of Washington to pick up children and get home. Citizens directed traffic at some gridlocked intersections, and workers and residents got little direction.

COG has built a $500,000 Regional Incident Coordination Communication System that can send immediate alerts to, and set up a conference call among, decision makers representing police, local governments, federal agencies, schools and transportation agencies.

But more must be done to persuade the public to have faith in the system, said Peter G. Laporte, D.C. Emergency Management Agency director. "We have to coordinate and communicate to everyone in the region," he said. "We have to exercise and drill -- we need to get down all the levels to the public."