From Bruce Schneier’s “Searching Bags in Subways“:
Final note: I often get comments along the lines of “Stop criticizing stuff; tell us what we should do.” My answer is always the same. Counterterrorism is most effective when it doesn’t make arbitrary assumptions about the terrorists’ plans. Stop searching bags on the subways, and spend the money on 1) intelligence and investigation — stopping the terrorists regardless of what their plans are, and 2) emergency response — lessening the impact of a terrorist attack, regardless of what the plans are. Countermeasures that defend against particular targets, or assume particular tactics, or cause the terrorists to make insignificant modifications in their plans, or that surveil the entire population looking for the few terrorists, are largely not worth it.