Cities and terrorism

Earlier I wrote about why cities make attractive targets for terrorism. One reason was that high population densities provide both anonymity (for the terrorists) and a lot of victims (i.e., on a subway or crowded street.) Now the folks at the Marginal Revolution point me to today's Wall Street Journal, which has a caveat: high density at a bomb site can generate more injuries but a lower overall death toll. This in turn means that finding a bomb early can escalate the death toll, because as the crowd thins the explosive power of the bomb is amplified. A depressing topic...

Early detection [of a bomb] can backfire because of the grisly fact that human beings act as human shields. "There is a trade-off between crowd size and crowd blocking," says Prof. Kaplan. 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...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.

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