"The September 11, 2001, (or 9/11) attacks on the high-profile workplaces of the World Trade Center (WTC) in New York City and the Pentagon in the Washington, D.C. area, made real the impact of an unexpected, or 'no-notice,' event in a metropolitan setting. The news coverage of the events of 9/11 showed thousands of people leaving the area of the WTC on foot. The evacuation from the borough of Manhattan included not only the typical traffic congestion expected in an evacuation in the United States, but thousands of pedestrians moving along with, or among, the vehicles. […] Evidence that large numbers of pedestrians may be part of an evacuation raised questions within the Federal Highway Administration (FHWA) about what actions are needed to manage pedestrian traffic during metropolitan evacuations and what FHWA can contribute in this area to ensure safe and effective movement of pedestrians while minimizing their impact on vehicular movement. The term 'no-notice metropolitan evacuation' here refers to an emergency evacuation taken as a protective action that is implemented for a portion of a densely built-up downtown area in a large city in the United States. […] The term 'pedestrian evacuation' generally refers to masses of people who leave a suddenly dangerous area in order to reach a safer place and do so on foot. For pedestrian evacuation to be of concern to transportation agencies, it entails the combination of masses of people on foot along with the corresponding congestion of the evacuation of others in private vehicles, always or at times moving along the same routes."
Federal Highway Administration Publication No. FHWA-HOP-07-066
United States Department of Transportation Federal Highway Administration: http://ops.fhwa.dot.gov/