Terrorism, Part II, By Steven D. Levitt, Freakonomics Blog, August 9, 2007
A lot of the angry responses make me wonder what everyday Americans think terrorists do all day. My guess is that they brainstorm ideas for terrorist plots. And you have to believe that terrorists are total idiots if it never occurred to them after the Washington, D.C., sniper shootings that maybe a sniper plot wasn’t a bad idea. The point is this: there is a virtually infinite array of incredibly simple strategies available to terrorists. The fact that it has been six years since the last major terrorist attack in the United States suggests either that the terrorists are incompetent, or that perhaps their goal isn’t really to generate terror. (A separate factor is the prevention efforts by law enforcement and the government; I’ll address that later.)
Ultimately, though, it strikes me that there are two possible interpretations of our current situation vis-à-vis terrorism.

One view is the following: the main reason we aren’t currently being decimated by terrorists is that the government’s anti-terror efforts have been successful.

The alternative interpretation is that the terror risk just isn’t that high and we are greatly overspending on fighting it, or at least appearing to fight it.