July 24, 2005
Deterrence and Subway Searches
Posted by Dave Hoffman

[Warning: The following post contains unrealistic economic models of the behavior of very evil people.]

Dan Solove at Prawfsblawg posted a few days ago about the new NYC subway search policy.  Key graf:

I'm dashing this post off quickly, so I won't explore the legality of this.  But regardless of the legal issues, this seems to me to be a very silly policy.  It is another big waste of money and time, as well as a needless invasion of civil liberties -- all for a cosmetic security benefit.  There are 4.5 million passengers each day on the NYC subways.  What good could a few random checks do?  The odds of the police finding the terrorist with a bomb this way are about as good as the odds of being hit by lightning. I doubt it will have much of a deterrent effect either.

Unlike Dan, I'm not an expert on privacy law.  But I do think some about deterrence, and I think he's underestimating the effect of random searches on public safety. 

Assume the police are executing a search policy (nicknamed "Ring of Steel") that results in 1000 random searches a day; this results (by my very rough calculation) in a 7% chance of being searched over the course of a year of weekdays.  We have to assume that are many intermediate steps necessary in conspiring to harm others; if the terrorists have to purchase, for example, washing machine timers, ordinary subway travel becomes hazardous to the plan.  (A cheap point: these are shorter odds than the 1 in 3000 chance of being hit by lightning over the course of your lifetime, although of course the odds of being searched on any given day are 1 in 4500).

Under these assumptions, Dan is probably wrong that the search policy will have little deterrent effect.  Thinking about the issue from a behavioral perspective, some keys are:

  1. Risk Aversion:  Terrorists are notoriously risk averse - they obey the law punctiliously until they attack. Thus, even a relatively minor risk of being caught will act as a very large deterrent, forcing terrorists to find other paths.
  2. Decision-making under uncertainty: Subway searches are going to be perceived (true or not) as targeted at certain ethnic minorities; the police, if they are smart, will also vary the number of searches per day.  These two effects together should make it hard for terrorists to intelligently evaluate the likelihood of being stopped on any given trip.
  3. Availability Cascades:  Again, smart police should perform searches in highly visible places, and trumpet "sweep" days (think of the IRS's press machine in March.)  The goal will be to convince folks that searches are much more common than they actually are, and to equate "going in the subway" with "possibly getting searched: better not bring illegal goods down there." [Maybe this sounds too big brother like for some.  Arguably, it would be better for society to pay the privacy cost of having lots of actual searches, so that folks could evaluate whether they think them to be worth it, rather than allowing the government to mind game the populace.]

Regardless, I think there is a good argument that terrorists, subject to human behavioral tics, are likely to vastly overestimate the likelihood of being caught and therefore be more deterred than rational terrorists (what a contradiction in terms that is!) would be.  So I have to disagree with Dan.  Subway searches can, if intelligently carried out, make it much more costly to plan and execute mass transit attacks.  Will terrorists then move on to other targets of opportunity?   Probably.  But forcing them to do so would be a victory.

Dan notes we should weigh the gains against the costs.  Fair enough. 

  1. Law enforcement costs:  Two kinds: (1) direct (for more police officers); (2) indirect (lost opportunities to catch crime elsewhere).  But there are also law enforcement benefits, primarily consisting of increasing the expected sanction for the transport of drugs and guns from one part of the city to another.  This should help the City to attack pan-neighborhood gangs.
  2. Civil Liberties: Again, several kinds: (1) constitutionally protected privacy (which Dan says he isn't going to talk about);  (2) constitutionally unprotected privacy (i.e.: the "private public space" so essential to living in a big city); and (3) equal protection.  The last of these really concerns me.  The privacy losses created by these searches will fall most heavily on poor and working class new yorkers, who, unlike my former law firm colleagues, will be unable to opt out of the system by regularly taking cabs/town cars instead of the subway.  This is a serious problem.  But, if it is true that searches deter attacks - and there is at least some reason to believe they will - maybe working class and poor New Yorkers will be willing to make that tradeoff? (One interesting data point is that the plan has taken flak from democrats running to oust Mayor Bloomberg for not going far enough!)

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Comments (8)

1. Posted by Chris on July 24, 2005 @ 22:44 | Permalink


Those are exactly my thoughts. National Security vs. taking my hiking pack on the subway; National security wins in my book.

Your point about the burden falling on lower class citizens might be slightly offset by the rise in cab usage, since many cabbies are poor minorities.

2. Posted by Aaron Wright on July 24, 2005 @ 23:22 | Permalink

The same argument could be made for border searches and the drug war. How well has that worked? Prices have dropped and more drugs flow into this country than ever.

Those who threaten our national security will calculate the odds of getting searched and, still wanting to effectuate an attack, will just increase the number of jihadists willing to sacrifice their lives. That or they will engineer smaller bombs, or attach them to their body like they do in Isreal.

Moreover, you mention some behavioral biases, what about the over-optimism bias? People have a tendency that they are less likely to be suceptible to a given risk then actually necessary. Therefore, a given terrorist would probably feel themselves less susceptible to the objective probability of getting searched.

Here's another - which I think just shows our societies irrational decision to allocate resources in ineffcient ways - the availability heuristic. People over-estimate the risks of memories and events more available, like terrorist attacks, than other risks of higher probability of harm.

Maybe instead of expending resources to remedy highly "available" remedies, we should focus on those less "available," alternatives, like improving our intellegence operations.

3. Posted by Rodney on July 25, 2005 @ 0:26 | Permalink

According to an article in the 07/25/05 issue of The New Yorker, NYPD has a $220 million a year counter-terrorism effort that rivals or possibly surpasses anything the FBI has been able to develop as far as man-power and scope. Public searches are realistically just a small piece of a larger set of comprehensive and less overt activities, including city-conducted foreign intelligence, terrorist counter-measures and community relations. So, I see both highly "available" and less "available" alternatives being tried in this situation. Lets see what works.

4. Posted by Jon Klick on July 25, 2005 @ 8:09 | Permalink

The ancillary deterrent effect on non-terror crimes is likely to be quite high, assuming that the cops used for the searches are working "extra" hours as opposed to being pulled away from other duty. Alex Tabarrok and I find large deterrent effects for "regular" crimes when DC increases its police presence when the terror alert level is raised: 48 Journal of Law & Economics 267 (pre-pub version available at http://papers.ssrn.com/sol3/papers.cfm?abstract_id=461280 )

5. Posted by SupremacyClaus on July 26, 2005 @ 6:10 | Permalink

Prof. Hoffman: The wisest tactic for the lawyer in the war on terror is silence. Judge Chertoff should resign, perhaps become an inside legal consultant.

Should a major terror incident take place, massive scapegoating of the profession will ensue. The propriety is moot.

The leadership of this "war" must come from the military or police. The lawyer should speak and technically, when asked a legal question by them. The lawyer should refuse to comment on policy, or to make any major decision. The exception might a former lawyer who is now full time military or police.

The lawyer has an affirmative duty to not have an incident take down the profession, its being an essential utility service.

Your remarks, however correct or proper, support this contention. This is not advocacy of censorship, but of self-restraint for survival.

6. Posted by SupremacyClaus on July 26, 2005 @ 6:13 | Permalink

Prof. Hoffman: One subject appropriate for lawyer discussion is the 4th Amendment. Inspection reveals a gun, fifty bras, tags on from Victoria's secret, no receipt, or a bag of marijuana. Suppressible?

7. Posted by Matt on July 27, 2005 @ 13:59 | Permalink

Regarding the subway searches, be sure and check out The Citizen's Guide to Refusing New York Subway Searches put out by the Flex Your Rights Foundation. It teaches subway riders exactly what they need to know in order to assert their rights when they encounter a subway search.

8. Posted by Eric Botticelli on December 3, 2005 @ 20:21 | Permalink

According to a 12/02/05 NYT article


Police Commissioner Raymond W. Kelly said through a spokesman, "Common sense prevailed." in response to a ruling by Judge Berman approving the random bag searches by the NYPD. Mayor Bloomberg also supported the ruling.

Kelly, Bloomberg, and Berman all need a lesson in common sense. If a person with a bomb is stupid enough to continue walking past the police checkpoint *and* unlucky enough to be searched, he will simply refuse, exit the station, walk 6 blocks and enter the next station, where there is no police checkpoint.

This policy does nothing to increase security and at the same time cuts deeply into the skin of the New Yorker's privacy, not to mention our rights under the fourth amendment.

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