Why Just Detecting Hidden Explosives May Not Cut Deaths:
Early detection 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.