A new book by researchers John Mueller and Mark G. Stewart asserts that the United States would need to thwart 1,667 Times Square-bombing type attacks per year in order to justify the enormous sums our nation has spent on domestic security since 9/11. That means officials would have to thwart four major attacks per day. The authors approached this enormous and multi-layered problem by doing a relatively straightforward cost/benefit analysis, looking at how much money is spent, and what tangible security benefits were gained as a result of spent dollars. Summarizing their methodology for Slate, they write:

A conventional approach to cost-effectiveness compares the costs of security measures with the benefits as tallied in lives saved and damages averted. The benefit of a security measure is a multiplicative composite of three considerations: the probability of a successful attack, the losses sustained in a successful attack, and the reduction in risk furnished by security measures. This product, the benefit, is then compared to the cost of the security measure instituted to attain the benefit. A security measure is cost-effective when the benefit of the measure outweighs the costs of providing the security measures.

The authors point out that the chances for an American of dying in a terrorist attack are about one in 3.5 million per year. On the other hand, the chances that you'll die in a car accident are about one in 6,500 per year. Given this incredible discrepancy, and the huge sums that are spent on so-called 'security' measures every year in the United States, the authors urge that public funds be reallocated. The numbers don't lie.

Risk reduction measures that produce little or no net benefit to society or produce it at a very high cost cannot be justified on rational life-safety and economic grounds: They are not only irresponsible, but, essentially, immoral. When we spend resources to save lives at a high cost, we forgo the opportunity to spend those same resources on regulations and processes that can save more lives at the same cost, or even at a lower one. Homeland security expenditure invested in a wide range of more cost-effective risk reduction programs like flood protection, vaccination and screening, vehicle and road safety, health care, nutritional programs, and occupational health and safety would likely result in far more significant benefits to society.

For more information on US 'security' spending post-9/11, check out this NYTimes infographic.