What is clear, however, is that no single sensor technology can be expected to find all threats with acceptable accuracy, so an array of sensor technologies will need to be developed and used together in a reliable, networked (“sensor fusion”) manner whereby each sensor can crosscheck the validity of others. Such crosschecking can help reduce false alarms and the need for inconvenient and costly follow-on searches, such as manual baggage inspections.

In general, all detectors—whether they sense explosives, say, or radiological materials—need to be made more accurate for use in transportation modes, where an excessive rate of false alarms can wreak havoc. They must also be made smaller, more affordable, and capable of operating at greater range. These latter requirements are particularly important if detectors are to be deployed strategically in the surface transportation modes.

Monitoring and Mitigation

Knowing when a hostile attack is under way, diagnosing it quickly and accurately, predicting its course, and mitigating its harmful effects are crucial capabilities that research and development can help provide.

Monitoring is essential to all these crisis-management functions. Indeed, the use of FAA’s air-traffic management system to ground aircraft on September 11 demonstrated how existing traffic operations and control systems can be used to detect terrorist attacks in progress and help manage the crisis. The fast and decisive actions taken by local traffic control centers to prevent commuter and subway trains from passing under the World Trade Center may have saved hundreds of lives.

Another example of monitoring capabilities that are not yet available but that could prove crucial in transportation settings is the development of real-time sensors to rapidly detect a wide variety of chemical agents. In a busy transportation environment, rapid recognition of a threat is critical to ensure appropriate response. A prerequisite for the development of such sensor systems is baseline information on the background chemicals in facilities such as subway systems and airport terminals, especially to ensure that sensor systems are designed to balance the risks associated with false positive and false negative readings. On the one hand, excessive false alarm rates are a major concern for transportation operators, lest localized service disruptions regularly propagate across an entire network, eventually causing the alerts to be ignored and alarm systems to be turned off. On the other hand, a single missed or neglected alarm runs the risk of exposing thousands of people to deadly agents and postponing effective emergency response. An appropriate balance must be struck between such risks, requiring risk modeling and human factors assessments.

Research on architectural features, materials, and construction methods to harden transportation facilities has the potential to mitigate the effects of blasts. Research on mitigation could also be useful in protecting structures from earth-

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