IN THE AFTERMATH of the terrorist attacks in London, the District's decision to test downtown emergency evacuation routes after Monday's Fourth of July fireworks display doesn't seem to have been such a bad idea. Earlier in the week, security experts faintly praised D.C. officials for attempting to measure the city's ability to steer large gatherings of people out of downtown. Missing in Monday night's exercise, critics said, were the chaotic and stressful conditions that are part of a real emergency. As the London bombings brought home, it is nearly impossible to anticipate, let alone be ready for, an attack that causes mass casualties. Nonetheless city officials gained valuable experience in moving large numbers of cars and pedestrians -- experience that will come in handy if and when the real thing comes along.

The alternative, clearly unacceptable, was for the city to pass on the opportunity to test new evacuation routes and leave everything to chance. Instead, Dan Tangherlini, director of the city's Department of Transportation, which coordinated the test, opted to use the occasion to pinpoint evacuation problems. "What we have been doing in the way of testing is war-gaming stuff and talking it through," he told The Post's Paul Schwartzman. As a result, city officials, as well as Thomas Lockwood, director of the Office of National Capital Region Coordination for the Department of Homeland Security, gained information that should prove useful under authentic emergency conditions.

Return to the London experience or the attacks in Madrid, Casablanca, Bali and Istanbul in the past three years. Those sites had two things in common: crowded public places and coordinated explosions. The District, already in the cross hairs of terrorists once, remains a likely target of terrorist actions on U.S. soil. District officials need not have a specific warning or word that a strike is on the way before they prepare the public for an emergency. That, in fact, is what the Fourth of July exercise was about. Monday night's evacuation did not come close to testing every circumstance that might develop in an emergency. But it was, to the city's credit, a necessary and good start.