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" At two miles long and roughly a city block wide, the National Mall in Washington, D.C., will become the world&amp;#039;s largest welcome mat on January 20. Those who wish to stand witness to the swearing-in of America&amp;#039;s first African-American president are being told to come--one and all.  But how will officials manage what some have speculated will become the largest gathering in American history? The U.S. Secret Service is coordinating a team of 58 public agencies whose overlapping jurisdictions are woven across the district like a spider web. At its center is the United States Park Police, who will patrol the mall from the edge of the Capitol grounds to the Lincoln Memorial. Altogether, 400 uniformed and undercover park officers will be stationed on the mall, in observation towers and helicopters and even atop the 500-foot Washington Monument, surveying crowd movements. Their main goal is clear: Keep order and be prepared to evacuate millions from this small, open space. With the help of experts and those on the scene, we look at challenges of and solutions to handling one of the largest crowds in all of history.  Millions at the National Mall: The Problem with Open Space   At the U.S. Park Police headquarters, Park Police Chief Sal Lauro points to a large map on the wall indicating where he plans to position people on the National Mall.   Unticketed spectators will be directed into three rectangular sectors between 4th street and the Washington Monument, each of which will be filled from the back until they reach capacity. &amp;quot;We&amp;#039;re going to keep letting people in until it reaches that point where we don&amp;#039;t feel it&amp;#039;s safe to let any more in,&amp;quot; Lauro says. At that point spectators will be able to flood onto the mound surrounding the Washington Monument and eventually back to the wooded areas around the Lincoln Memorial, areas onlookers typically never go to for presidential inaugurations. Several JumboTron screens will be broadcasting the ceremony to spectators along the mall.  One of the biggest challenges for controlling the crowd at the Mall is the absence of natural barriers or tiered levels, Lauro says. &amp;quot;A stadium or concert hall has fencing that can control the people,&amp;quot; Lauro says. But the National Mall has none of that. &amp;quot;A crowd could pack in as tightly as they can force themselves in, which creates problems for us,&amp;quot; he says. Pack enough people into an open space without barriers to help traffic flow, and the likelihood of injuries or fatalities in the case of an emergency rises significantly.  &amp;quot;A huge empty area is the worst place for controlling crowds,&amp;quot; says Zurich Anders Fredrick Johansson, a crowd expert from the Swiss Federal Institute of Technology. You need to make sure there are no significant bottlenecks in the area, he says, and put in place barriers that separate the people and organize them into kinds of compartments. The police are doing just that. Barriers are being shipped into D.C. from all over the country. At least 30 miles of waist-high barriers and fencing have been installed throughout the mall to direct traffic and contain the sprawl, according to Jason Vaughn of the Pittsburgh-based Tamis Corporation, which is supplying the barriers. The crowds are thereby forced to enter and exit in a more orderly manner. But once they&amp;#039;re in, what happens if a few trouble-makers start protesting, or if it rains or, in the worst case scenario, disaster strikes? That&amp;#039;s where the crowd experts can help.  The Science of People: Crowds as Systems  Watching crowds from a height is a lot like looking down onto a colony of ants. Aerial videos of crowd disasters typically exhibit recurring patterns in the minutes leading up to deadly surges, says Johansson, who has published studies on the 2006 stampede that killed at least 345 hajj pilgrims near Mecca in Saudi Arabia.  What begins as a smooth flow of people can quickly turn into stop-and-go movements characteristic of highway traffic jams. This is the first warning sign. Like any orderly system, once you see an disruption or impediment on the flow, turbulence (science-speak for chaos) is not far away. In the case of a crowd, this is where panicked people begin to thrust away from each other in all directions, leaving some with no place to go and, in the worst case, vulnerable to being trampled.  Dr. Paul M. Torrens at Arizona State University uses sophisticated computer models to predict crowd behavior and spot problem spots in a given landscape. The agents in Torrens&amp;#039; computer models have a realistic field of vision that can change according to personal attributes such as speed, height, age and even personality traits and predilections (the meanderer versus the goal-line oriented). People are able to scan subconsciously for collisions, says Torrens. His models are a bit more rudimentary--the avatars in them avoid physical collisions at all cost. Still, their myopic viewpoint gives insight into how a person will act in a panicked crowd. With the right barriers in the right places, a panicked crowd is given nowhere to go. Officials on the ground take insight from experts such as Torrens in order to implement careful urban planning. It is the preplanned area, after all, that will save a crowd from its inability to see the whole situation. In Case of an Emergency: The Evacuation Plan  Regardless of the number of people, police and transportation officials are ready to implement one of the largest evacuations in history. In the event of an emergency, park and district police assist the District DOT in coordinating a comprehensive &amp;quot;pedestrian-centric&amp;quot; evacuation.  This is where it gets tricky. The lay of Washington is pinched at the bottom by the Potomac and Anacostia Rivers, meaning any exit strategy must point north.  &amp;quot;Because of the geography, if you&amp;#039;re trying to evacuate just south of the mall area, there are limited opportunities to get people out,&amp;quot; says Soumya Dey, the district&amp;#039;s deputy assistant director.  Extensive road closures surrounding the inauguration mean spectators will all be leaving by foot, at least initially. Outside the inauguration security perimeter, pedestrians will be able to board DC Metro trains and buses, a system that can transport up to 120,000 riders per hour. The rest will need to board chartered buses or personal vehicles, or just keep walking. They&amp;#039;ll follow one of 19 arterial evacuation routes radiating diagonally out of the city. Transportation officials will synchronize all of the outbound traffic lights to 240-second cycles and monitor traffic flow on 150 street cameras.  &amp;quot;What that enables you to do is flush traffic out of the district in a pretty efficient manner,&amp;quot; Dey said. &amp;quot;We are setting up restrictions along every major intersection along the corridors to minimize friction with the side streets.&amp;quot; It&amp;#039;s a routine D.C. officials are comfortable with after testing it at the past four 4th of July firework shows on the Washington Mall. Still, it&amp;#039;s something they&amp;#039;re hoping not to use Tuesday afternoon. "^^<http://www.w3.org/2001/XMLSchema#string>
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