Some people rationalize long waits in airport lines by thinking, "if this is the cost of safety, I can bear it." Problem is, at some point the lines themselves, outside the 'secured zone,' can become an attractive target. So there is some margin at which rigorous entry-control, because of the lines it creates, decreases overall safety.

(For another example of a scale vs. security tradeoff, see Brad Templeton wondering "Do we need security on small planes?")

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