From Bruce Schneir's CRYPT-GRAM Newsletter and John Mueller:
In an excellent paper by Ohio State political science professor John Mueller, "The Quixotic Quest for Invulnerability: Assessing the Costs, Benefits, and Probabilities of Protecting the Homeland," there are some common sense premises and policy implications.
"1. The number of potential terrorist targets is essentially infinite.
"2. The probability that any individual target will be attacked is essentially zero.
"3. If one potential target happens to enjoy a degree of protection, the agile terrorist usually can readily move on to another one.
"4. Most targets are 'vulnerable' in that it is not very difficult to damage them, but invulnerable in that they can be rebuilt in fairly short order and at tolerable expense.
"5. It is essentially impossible to make a very wide variety of potential terrorist targets invulnerable except by completely closing them down."
The policy implications:
"1. Any protective policy should be compared to a "null case": do nothing, and use the money saved to rebuild and to compensate any victims.
"2. Abandon any effort to imagine a terrorist target list.
"3. Consider negative effects of protection measures: not only direct cost, but inconvenience, enhancement of fear, negative economic impacts, reduction of liberties.
"4. Consider the opportunity costs, the tradeoffs, of protection measures."
The whole paper is worth reading.