ON New Year’s Eve 1999, Fred Siegel writes in “The Prince of the City,” his new book about Rudolph W. Giuliani’s New York, authorities feared that terrorists would seize on Y2K computer glitches to strike in Times Square. In response, the National Guard was secretly mobilized in Brooklyn “as part of an emergency plan for evacuating Manhattan.” As midnight came and went, the computers hummed on, the celebration proceeded flawlessly and officials concluded, Mr. Siegel notes with a tinge of sarcasm, “Gotham was ready for a future emergency.”

In fact, no plan existed that night for evacuating all of Manhattan. The guard unit at the Brooklyn Navy Yard consisted of about 100 troops and 50 trucks, and their mission, in the event of an attack, was limited to ferrying the injured out of Times Square.

Today, four years after the Sept. 11, 2001, attacks and in the wake of Hurricane Katrina, there is still no single plan to evacuate all of New York, which virtually no one believes is possible. If New York’s anthem was about fleeing the city instead of its lure, its lyrics might read: “If you can make it out of here, you can make it out of anywhere.”

Just imagine trying to move more than eight million New Yorkers – including the high number of people without cars – through streets that are clogged on an ordinary day and then through the tunnels and over the bridges that connect New York’s islands to the mainland and to one another. “It would not be easy and it would not be pretty,” said Jerome M. Hauer, the city’s former emergency management director.

History offers little comfort. For example, on Nov. 25, 1783, British troops began their retreat from New York (a day still celebrated in some Irish neighborhoods as Evacuation Day). It took them a full month.

During World War II, civil defense focused on air raid shelters, but the advent of radioactive weapons in the cold war inspired proposals to evacuate people by boat (after a test-run by a flotilla of 20 ferries, barges and tugboats up the East River in 1951, officials figured 100,000 an hour could be spirited away for six hours; then the flow “would taper off for lack of equipment”). There were also plans to construct atomic-proof shelters for 1.5 million beneath city parks, in underground stations in Washington Heights and along a Second Avenue subway bored through rock, and to build two cross-town expressways to speed the escape from Manhattan.

Even so, a mayoral panel concluded in 1955 that only a million people could be moved from the worst danger zones within an hour. “Until more efficient use of transportation and more than one hour’s warning can be assured,” the panel said, “about three million people, or 37 percent of the city’s eight million population, might be balked in any attempt to escape the target area except by walking.”

In 1966, the city’s civil defense director, Timothy J. Cooney, admitted the obvious: “If a nuclear bomb fell in our midst, civil defense would be an academic question.”

Today, the city appears to be better prepared than ever for disasters, especially natural ones like hurricanes (a Category 5 hurricane has apparently never hit the city head on). Officials have maps of escape routes from vulnerable neighborhoods near water to 23 reception centers and public shelters, the ability to mobilize fleets of buses, and a keen sense of contingencies (like knowing when bridges would have to be closed because of high winds and when subway and car tunnels might flood).

“It’s very important to have a sense of order if you have an evacuation and we are able to mass 37,000 cops in the neighborhoods that need it, where people are poor or infirm,” said Police Commissioner Raymond W. Kelly. Still, as the city’s Household Preparedness Guide says: “Evacuation is used as a last resort.”

Joseph F. Bruno, the emergency management commissioner, said the city is prepared to move from 400,000 to two million people from the path of a hurricane – a challenge made a little less daunting by advance warning, knowing which flood-prone areas to evacuate and identifying how many poor, elderly, disabled and non-English speakers live there. Since 9/11, with its hellish communications breakdowns, New York officials said they have also vastly improved their ability to communicate with the public by radio and television and, to a lesser extent, with each other.

Still, much of the planning assumes that people already know what to do (the city’s preparedness guide is available online at nyc.gov/readyny and two million copies have been distributed in eight languages), or would telephone the city’s information line, 311, which can handle only so many calls (about 178,000 two years ago on the day of the blackout).

“Would it be difficult to move two million people? Absolutely,” Mr. Bruno said. “I hope we never have to do it.”

Which means evacuating eight million would be beyond difficult. “We have plans for area evacuations, and if you take them to their logical conclusion an area could be the entire city of New York,” Mr. Bruno said. “Those are doomsday type things, a nuclear attack. We’re definitely not throwing our hands up. But it would be a catastrophic event that would be extremely difficult for New York City to have to deal with.”

How long would it take to virtually empty the city? “I wouldn’t even hazard a guess,” Mr. Bruno replied.

Mr. Hauer, now a consultant in Washington, said evacuating the whole city would not be impossible, but would be fraught with nightmarish challenges, like rescuing people from hospitals and nursing homes and reversing traffic flows. “It’s a matter of where do you put all those people when you get them out of Manhattan,” he said.

And, in a nuclear explosion, Mr. Hauser added, there’s is the danger of radioactivity. “Rescue workers might, without any idea of protection, at the end of the day choose to stay out of the plume and I can’t blame them,” he said. “Obviously, there’d be a lot of self-evacuation.”

That’s more or less what happens after work every weekday when half the borough’s daytime population – nearly 1.5 million commuters – leaves Manhattan to return home. Perhaps there’s some comfort in remembering that, except for the stragglers, most eventually make it.