...airports that day. A critical component in aviation security systems is the screening of passengers prior to boarding aircraft. Developing strategies to effectively and efficiently screen passengers, as well as allocate and utilize security screening devices, can be quite challenging. Moreover, it can be very difficult to measure the effectiveness of such systems after they are in place.

There are two basic approaches to aviation security screening: uniform screening and selective screening. Uniform screening subjects every passenger and their baggage to identical security screening procedures. The argument for uniform screening is that anyone could pose a risk, and hence, all passengers should be screened using the most effective technology and procedures available. The 100% baggage screening mandate, which requires all checked baggage to be screened by a federally certified explosive detection technology (effective December 31, 2002), is a move towards uniform screening (Mead, 2002). One disadvantage of uniform screening is that it can be very costly to apply expensive new technologies to every passenger or bag. Butler and Poole (2002) and Poole and Passantino (2003) suggest that 100% checked baggage screening is not cost effective, and suggest that creating multiple levels of security for screening passengers may be more effective than treating all passengers the same.

Selective screening, the alternative to uniform screening, selectively applies security technologies and procedures to a subset of passengers. The argument for selective screening is that most passengers do not pose a risk, and hence, expensive security technologies need not be used on all passengers. Selective screening subjects passengers perceived as high risk to closer scrutiny by screening them and their baggage with more sensitive and accurate technologies and procedures, while passengers perceived as low risk are subjected to lower levels of scrutiny. This approach requires that a prescreening system perform a risk assessment of each passenger prior to the passenger's arrival. A weakness of selective screening is that it can assign an incorrect degree of risk to a passenger, either by error or through "gaming" of the system by a terrorist. McLay et al. (2005) conclude that using expensive and accurate baggage screening technologies on a small proportion of passengers is warranted only if there is an effective prescreening system in place.

In the aftermath of September 11, 2001, the Transportation Security Administration (TSA) developed a prescreening system called Secure Flight for use with a selective screening system. Secure Flight is a modification of CAPPS II (which was an updated version of CAPPS, the computer-aided passenger prescreening system that was originally developed and deployed by the Federal Aviation Administration in the late 1990s). Secure Flight prevents an extremely small number of passengers from flying, based on Federal watch lists (TSA, 2004). The remaining passengers are assigned to one of two security classes. They are either cleared of posing a risk (i.e., these passengers are labeled nonselectees), or they are not cleared of posing a risk (i.e., these passenger are labeled selectees). Each of these two classes is defined by a preassigned subset of devices and a procedure through which passengers are processed prior to boarding an aircraft.

Each device is an aviation security technology or procedure used to identify a threat. Each device screens passengers in one of three ways: by screening checked baggage, carry-on baggage, or passengers. At present, all checked baggage is screened for explosives either by an Explosive Detection System (EDS) or an Explosive Trace Device (ETD). All passengers are screened with a magnometer and their carry-on baggage is screened with an X-ray machine. Each device has an associated capacity, the upper bound on the number of passengers or bags that a device can screen in a given amount of time. Selectees and their carry-on baggage are differentiated from nonselectees by undergoing hand searches by airport screening personnel. In some airports, selectees are screened with hand wands or explosive trace portals and their carry-on baggage is screened by trace devices.

One weakness of using an automated prescreening system, including Secure Flight, is that such systems can be gamed through extensive trial-and-error sampling by a variety of passengers through the system (Barnett, 2001; Chakrabarti and Strauss, 2002). Martonosi and Barnett (2003) note that CAPPS II may not substantially improve aviation security if the prescreening procedures for each type of passenger are not effective. Barnett (2004) suggests that CAPPS II may only improve aviation security under a particular set of circumstances and recommends that CAPPS II be transitioned from a security centerpiece to one of many components in future aviation security strategies. The TSA describes Secure Flight as a critical component in their layered aviation security system, which includes reinforced cockpit doors, bomb-sniffing dogs, and deploying Federal air marshalls on numerous flights (TSA, 2004).

Several integer programming and discrete optimization models have been formulated to describe aviation security problems. Jacobson et al. (2001) provide a framework for measuring the effectiveness of a baggage screening security device deployment at a particular station, where a station is a set of airport facilities that share security resources. Jacobson et al. (2003) introduce three performance measures for baggage screening security systems and introduce models to assess the security effect for single or multiple stations. The models developed consider using baggage screening devices (EDSs) to screen selectee checked baggage and to possibly screen a proportion of nonselectee checked baggage. The three performance measures considered are to maximize the utilization of baggage screening devices, maximize the number of flights that are covered (i.e., all selectee baggage on the covered flights have been screened), and maximize the number of passengers on the covered flights. Jacobson et al. (2005a) formulate problems that model multiple sets of flights originating from multiple stations subject to a finite amount of security resources. These problems consider three performance measures. Examples illustrate strategies that may provide more robust device allocations across all performance measures. Virta et al. (2002) and Jacobson et al. (2005b) consider the impact of originating and transferring passengers on the effectiveness of baggage screening security systems. In particular, they consider classifying selectees into two types; those at their point of origin and those transferring. They construct integer programming models for problems that consider multiple sets of flights originating from multiple airports. This is noteworthy since at least two of the hijackers on September 11, 2001 were transferring passengers.

Other research has focused on the experimental and statistical analysis of risk and security procedures on aircraft. For example, Barnett et al. (2001) performed a large-scale 2-week experiment at the nation's airports to test which costs and disruptions would arise from using positive passenger baggage matching, a security procedure that removes a passenger's checked baggage from an aircraft if the passenger did not board the flight. They concluded that using positive passenger baggage matching results in an average delay of 1...

NOTE: All illustrations and photos have been removed from this article.



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