The sprawling crowds on the Mall for Monday's July 4 fireworks display will participate in the first major test of Washington's downtown emergency evacuation routes since the Sept. 11, 2001, terrorist attacks, officials said yesterday.

Starting 15 minutes after the fireworks show ends, police officers are to direct tens of thousands of pedestrians and motorists leaving the Mall to seven evacuation routes, in which green and red traffic signals will be extended to four minutes. The test will last 45 minutes.

"This is a major step forward in emergency transportation planning, which we hope we never have to use for real," Dan Tangherlini, the District's director of transportation, said in a statement, adding that the test "will show how traffic and pedestrians are affected and how our transportation systems work in an emergency."

The routes in "Operation Fast Forward" include Constitution Avenue between 15th and 23rd streets NW; Independence Avenue between Third Street and Washington Avenue SW; and 14th Street between Pennsylvania Avenue NW and C Street SW.

Department of Transportation officials will observe the evacuation from the command center in the Frank D. Reeves Municipal Center on 14th Street NW, watching televised images from some 50 traffic cameras at the intersections. To replicate emergency conditions, officials will alter the timing of the traffic signals using laptop computers.

Douglas Noble, the department's chief traffic engineer, said the agency will view the test as a success if "we have an uneventful and quick exiting for people leaving the event. We will have collected information in terms of how well this works in getting people out of the city."

Law enforcement and transportation officials across the region have struggled since 2001 with the logistics of evacuating Washington during a terrorist attack. Concluding that the region's roads and transit network cannot accommodate an evacuation of the entire city, officials have focused on moving smaller groups from specific locations.

Metro estimates that more than 300,000 spectators traveled underground to last year's Fourth of July celebration, which was marred by rain. More than 500,000 people rode Metro to the fireworks display the year before.

About 125,000 vehicles -- more than a sixth of the number that drive city streets during a weekday rush hour -- are expected to travel downtown for the event, said John B. Townsend II, a spokesman for AAA Mid-Atlantic.

Recalling the panic downtown on Sept. 11 as people tried to leave the city, Townsend said that conducting the test on a holiday weekend is an effective way to "approximate what you have on a good workday in D.C."

The test would help motorists leave the area more quickly, he said, as well as demonstrate how officials can improve evacuation plans.

"It's a litmus test for how the situation will work during an emergency," he said.

The list of downtown evacuation routes also includes Ninth Street between Pennsylvania Avenue NW and Interstate 395; South Capitol Street between Washington Avenue and M Street; Washington Avenue between Independence Avenue SW and South Capitol Street; and Third Street between Constitution Avenue NW and Independence Avenue SW.