Of all the what-ifs, will-theys and how-abouts surrounding a terrorist attack, it is perhaps the most difficult question.

If the worst happens -- a chemical, a radiological or biological strike -- and local officials order a mass evacuation, how do you get the hundreds of thousands of workers, residents and tourists out of Washington, while emergency crews are rushing to get in?

The answer, in all likelihood, is that you don't.

A large-scale evacuation would require an almost unfathomable level of choreography and coordination among local governments, their first responders and their federal authorities. Street lights would have to be synchronized to keep the torrents of traffic from backing up, school buses and taxis conscripted into service, major thoroughfares closed to vehicles for pedestrians and bicyclists.

Officials could take roads that normally run in two directions and make traffic flow one way. For example, the eastbound lanes of Interstate 66 could be turned into westbound lanes, so that the entire highway would carry traffic out of the area.

Officials also would have to summon the almost preternatural ability to calm thousands of panicked people on the run.

That's why Doug Bass, the emergency management coordinator for Fairfax County, called large evacuations "the least favorable option." If there are pathogens in the air, he said, "A car offers little or no protection." And people fleeing blindly "may be going right into harm's way" and blocking emergency crews trying to get where they need to be, Bass added.

While officials continue to draw up evacuation routes, test communication systems and plan for virtually every imaginable scenario -- what if Metro had to be shut down? -- they warn that sometimes the best route to safety can be staying put. Human brains might be wired to run from danger, but officials said that could be the worst thing to do, especially when thousands of others have the identical idea.

The Metropolitan Washington Council of Governments' evacuation plan, released on the anniversary of the Sept. 11, 2001, attacks, warns: "A major incident can lead to panic and spontaneous evacuation of an area far greater than necessary, resulting in gridlock on the transportation network that compounds the dangers and difficulties in responding to the initial incident."

So the first thing to do, whether you are around the corner from the White House or in the suburbs, is to listen for instructions, officials said. Otherwise, the area's network of roads and transit, which are routinely clogged during rush hour, could be paralyzed.

"We have only a certain amount of capacity on our highways and transit systems," said Ronald Kirby, director of transportation planning for COG. "If the whole city goes to Metro, it's not going to work. What we really need to think about is how we minimize travel that's not absolutely necessary."

If there is a decision to evacuate, authorities say they will broadcast it. So in the event of an attack or disaster, stay tuned to the media, and make sure you have a battery-powered radio in case the power goes out. Local leaders said one of the first things they'll do is take to the airwaves -- and, if necessary, go knocking on doors -- to explain how best to respond.

"What's critical is to get information out to people to let them know they are safe where they are, and, if so, to stay put," Kirby said.

"For those who are not safe, we need to tell them the best way to get out -- and where not to go. Perhaps the 14th Street bridge is out. Who knows?"

The myriad unknowns make it impossible to foresee every detail of an evacuation, officials said. For all the preparation underway, the reality is that many plans will be made on the fly.

"Our responses are incident-specific," said Jo'Ellen Countee, spokeswoman for the D.C. Emergency Management Agency. "It all depends on what is going on."

There are, however, some basic things people can do to be ready should the order to evacuate ever come:

* Keep at least half a tank of gas in your car at all times.

* Know the quickest routes out of the area.

* Take a flashlight and extra batteries.

* Take a first-aid kit, maps, bottled water, sleeping bag, clothes and food.

* Before leaving home, close and lock doors and windows and unplug appliances.

* Know how to shut off your home's electricity, gas and water in case you are asked to do so.

The District has more than a dozen designated evacuation routes, each with signs, that lead out of downtown like wheel spokes.

Pennsylvania Avenue would act as a dividing line. Those above it would travel in northerly directions; those below would head south. No vehicles would be allowed to cross the boulevard, which would be used as a main evacuation route.

Uniformed police officers would fan out to 70 major intersections to keep traffic moving. Lights would be synchronized, and officials would monitor road conditions with cameras. Tow trucks would descend on emergency routes to remove cars blocking the road and clear debris.

The plan developed by COG calls for buses to transport residents from designated "staging" areas and ship them to safety. The buses could take the place of Metro, should the subway be shut down, or transport people to different Metro lines that are working.

Officials also could mandate "emergency car pools," where "restrictions could extend to allowing one car per family . . . or mandating that only vehicles with four or more persons are allowed access to major evacuation routes," according to the council's plan.

And local governments could ask residents for help as well.

"Remember during the snowstorm when the call went out for four-wheel-drive vehicles so we could get doctors and nurses to work?" said Mernie Fitzgerald, a Fairfax County spokeswoman.

But for all the drills, the plans are still largely untested, and no one can say for sure exactly how well a major evacuation would work. And most officials say a massive evacuation, ordered by the government, is not very likely.

"It's a rare occasion," said Pete Piringer, a spokesman for Montgomery County Fire and Rescue Services.

Most evacuations are a just a few blocks' worth of people, who are transported a few miles away to a nearby shelter. And sometimes it's best not to go anywhere at all, even if you're a long way from home.

"People can stay in their offices overnight," Kirby said. "That may be better than going out into a dangerous situation."