At the more local level, a quarter or more of the workers in some large cities commute by public transportation, which has come to shape some urban centers, most notably on the East Coast. The U.S. Postal Service delivers mail to every household in the United States and most businesses, totaling some 135 million addresses. The highway system pervades the lives of Americans, who use motor vehicles for most daily activities and for much of their longer-distance vacation travel.

Highways are also used by emergency responders, and both the highway and public transportation systems are vital security assets to evacuate people in a crisis and move critical supplies and services. Consequently, disruptions to transportation networks can have far-reaching effects not only on transportation operations but on many other interconnected functions and activities.

Certainly, undermining the ability of terrorists to attack in the first place is a national imperative. Should these efforts fall short, however, the transportation sector must be prepared to defend itself. The above characteristics reveal the great difficulty, indeed impossibility, of defending each potential target or perceived vulnerability one by one. The transportation sector is simply too spread out, diverse, and open—by necessity—for such a defensive approach to work. This does not mean that little or nothing can be done to counter terrorism. Sound security measures can do a lot; for instance, they can confound and deter terrorist operations, increase the likelihood of the terrorists being detected and intercepted, keep casualties and disruptions to a minimum, and reduce panic and reassure passengers in a crisis.⁶

What the characteristics of the transportation sector do suggest is the need for a coherent and systematic approach to security. In particular, such an approach should be shaped by (1) well-designed, layered security systems, (2) the adaptive, opportunistic, and dual use of security technologies and techniques, and (3) broad-based and unconventional thinking on terrorist threats and responses.

Transportation security can be best achieved through well-designed security systems that are integrated with transportation operations. The concept of a layered security system, in which multiple security features are connected and provide backup for one another, has a particular advantage. Perfect execution by each element in the system is not crucial, because other elements can compensate