District officials have briefed businesses this week about plans to evacuate downtown Washington and urged private employers to review emergency procedures for workers and vital operations.

The warnings are meant to prevent a repeat of the confusion and gridlock that paralyzed the capital after the Sept. 11, 2001, terrorist attacks. Property owners were advised to be ready to shelter building occupants for at least four hours. They also should keep surveillance videotapes for at least 10 days and be prepared to secure ventilation systems, officials said.

In addition, businesses that could fall within expanded security perimeters around places such as the White House and U.S. Capitol are bracing for the possibility that the movement of goods and employees may be interrupted for indefinite periods.

The warnings, delivered in public settings and in private sessions, followed the Bush administration's decision Friday to raise the national terrorist threat index to Code Orange, the second-highest level in a five-color system put in place last year. Across the city yesterday, the heightened alert triggered meetings as businesses tested their preparations and reminded apprehensive employees and a jittery public about procedures for worst-case scenarios.

Transportation and emergency workers have prepared over the last 17 months to deal with the challenge of directing the movements of about 1.75 million workers and tourists who join the District's resident population of 572,000 on an average workday.

The D.C. emergency plan could be invoked to clear affected areas of the city in case of a natural disaster or an attack involving a radiological device or other weapon of mass destruction.

In presentations and training sessions led by D.C. Police Chief Charles H. Ramsey and others, authorities have told management of hotels, office buildings, law firms and other businesses that the city has identified 14 routes to be used to empty downtown in an emergency.

The plan calls for Pennsylvania Avenue NW between Rock Creek Park and the U.S. Capitol to form a dividing line. Motorists north of the avenue would evacuate to the north, west and east. Motorists south of the avenue would evacuate to the south, west and east. No vehicles would be permitted to cross the avenue.

To keep exit routes clear, traffic signals at key intersections would be reset to four-minute cycles, with maximum green-light time. Uniformed police officers would staff 70 critical intersections to speed traffic. Signs marking corridors to Interstate 495, the Capital Beltway, have been posted for months throughout the region.

Separately, the federal government has developed an emergency protocol for ordering the evacuation of 180,000 federal workers in the city and possibly others across the country. The decision would be made by the heads of the Office of Personnel Management, the General Services Administration and the Federal Emergency Management Agency.

Coordinated contingency planning began long before the latest warnings, but as the nation has moved closer to war with Iraq, regional officials have stepped up efforts.

"Obviously, in the last few weeks, we have been getting ready for the possibility of war," City Administrator John A. Koskinen said. He added, "Part of our emergency planning process has been to develop closer links to the private sector, including business."

Federal security officials said they are sensitive to concerns that new measures may cause confusion and emphasized that they are working with local officials and other constituents.

After the 2001 terrorist attack on the Pentagon, the federal government immediately released workers, while authorities shut down major bridges and roads. Authorities also barred vehicles and pedestrians from a varying area around the White House complex, causing traffic jams for hours. Today, E Street NW remains closed and truck traffic has been banned from 17th Street NW.

Secret Service spokesman Mark Connelly declined to discuss specific plans. He said that White House security needs maximum flexibility and that plans should not be disclosed to terrorists. He said, "Obviously, when we have to make adjustments to the security perimeter . . . we would work with affected parties to minimize any negative impact."

At the Capitol, U.S. Capitol Police have increased the deployment of officers carrying heavier arms, such as shot guns, automatic rifles and submachine guns, Chief Terrance W. Gainer said.

Congressional officials are putting in place a perimeter security plan, replacing concrete barriers, police vehicles and other temporary barricades with permanent measures around Capitol Square and the House office complex. Crews also have imbedded hydraulic "pop-up" barriers beneath steel plates across Independence Avenue and enforced a truck ban on Constitution and Independence avenues next to the Capitol.

"At our present alert level, there is no additional action anybody has to take," Gainer said. "But clearly, one thing we know is, terrorists do a lot of surveillance, and ultimately, before they do their attack, they come back and look at their site picture. If that picture has changed, that deters them. We are trying to make ourselves look different, act different, and not be routine in what we do."

He added: "In an emergency, every department is going to have to do what it has to do to protect its people . . . [but] none of us are blind to the fact that closing major streets and intersections has an impact on safety."