the publicizing (but not the identification) of new but unspecified passenger screening procedures.
If deterrence is unsuccessful, the next line of defense is prevention, whether by denying access through physical means—guards and fences, for example—or by other methods of interception, such as passenger profiling, baggage inspection, and explosives detection.
A topic likely to generate much research and debate in the years ahead is how best to filter out the lower-risk users of transportation systems in order to focus security resources on anomalies and the higher-risk traffic. Advanced information technologies offer some promising tools for such identification and prescreening. What is needed, however, is a better understanding of the markers of risk, the kinds of data useful for identifying those markers, and how to interpret and use the results for detection and control purposes.
For example, the application of automated passenger prescreening systems may depend less on advances in biometrics, artificial intelligence, statistics, and computer hardware than on the kinds and quality of data that can be employed in these systems. Not only must the multiple, heterogeneous databases involved be accurate and compatible (both criteria present major challenges), but the right information must be extracted and combined. As an example, how can data on a traveler’s financial records, immigration status, legal history, demographic characteristics, and matches to traveling companions on the same flight be used to evaluate his or her security risk, and who will act on the results? Will new databases be created by the linking of various private and public data sources? And if so, how will the information be stored and protected, and who will have access to it and for what purposes? Research on numerous such issues is clearly required to help policy makers evaluate preventive measures.21
Yet another prevention-related need is for explosives detection systems that are sensitive to a wider range of materials. At the moment, many threats are not detectable; for instance, a pouch sealed in plastic and taped on a person’s body may not register with available screening devices. But new and emerging techniques could augment existing detection capabilities. For example, three sensor technologies that appear to hold promise for explosives detection are x-ray diffraction, which detects several types of explosives; microwave/millimeter-wave scanners, which penetrate denser substances; and nuclear quadrupole resonance, which identifies the chemical compositions of selected materials.22