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Sunday, July 10, 2005

Don't Run, it's a Bomb!

Edward Kaplan of the Yale University School of Management, and Moshe Kress of the Naval Postgraduate School reached counterintuative conclusions in a new study about suicide bombers as reported by the Wall Street Journal (free):
If, for example, a suicide bomber walked into a crowded plaza, "standoff" bomb detectors might well pick up an unambiguous signal. A terahertz imaging system could spy the telltale wires and explosives in 30 milliseconds, and ultraviolet-visible spectroscopy now in development could sniff out the trace vapors emitted by the ethylene glycol dinitrate in the plastique. Let's say the sensors alerted a security guard, who spotted the terrorist and yelled to the crowd, "Run, it's a bomb!"

In this scenario, the explosives-detection technology worked perfectly. An alarm sounded before a detonation. People were able to run or throw themselves to the ground. But when the bomber exploded, the casualty toll might not have been any less than if the sensors weren't deployed. Even worse, in some situations the intervention -- "Run!" or "Get down!" -- could lead to more casualties
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A large, dense crowd puts more people in harm's way, but "the probability of being exposed to a bomb fragment declines exponentially with the size of the crowd." As a crowd flees, there are fewer people near the bomber to absorb the fragments (as when a soldier falls on a grenade) and more people, unshielded, farther away. Simple geometry shows that you can hit more people at a radius 20 feet from a bomber than you can five feet from him.

"If the first ring of unshielded people is at a greater radius, there are more of them, and more will be hit," says Prof. Kaplan.

The same effect occurs if people throw themselves to the ground. That minimizes each person's exposed area, but also at the expense of decreasing human shielding. For bombs with 500 or more fragments (in Israel, 1,000 is typical), "hit the deck" can raise rather than cut casualties. If scores of people fall from an average height of five feet eight inches to 1.5 feet, the scientists calculate, casualties could rise as high as 50 from 37.
Kaplan and Kress published their paper in the Proceedings of the National Academy of Sciences.

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