in the United States some network segments have been pruned back in recent decades. In the Great Plains, for example, many rural roads have been abandoned, in part because of declining rural population densities in some areas and in part because of the increasing costs of maintaining older infrastructure such as bridges. As networks are rationalized, the remaining ones become more vulnerable because there are no alternatives in the event of failure or attack. This problem is especially apparent in the rail network, which has been drastically thinned as the system has modernized and become more cost-conscious, to the point that in some areas the network now lacks almost all redundancy.

Ports are especially vulnerable points in the nation’s transport network. Assessing the impacts of losing major port facilities to disaster and identifying potential alternative trade facilities should be two high-priority research topics. The ports of Los Angeles and Long Beach, for example, handle nearly one-quarter of U.S. total exports and 40 percent of all containerized cargo import traffic, a trade volume equal to $256 billion in 2005 (BST Associates, 2007). The importance of these ports to the national economy is further underlined by the fact that more than 60 percent of the cargo arriving there is destined for markets outside Southern California (BST Associates, 2007), and two-thirds of exports originate outside California (POLA/POLB, 2008).

The geographical sciences can also contribute to identifying the greatest points of vulnerability in the U.S. transportation network and document the impacts that would follow should mobility through those vulnerable points be lost. A recent National Research Council report, Potential Impacts of Climate Change on U.S. Transportation (NRC, 2008b), called attention to the vulnerability of transportation infrastructure to climate change, concluding that the most vulnerable places are likely to be in coastal regions. That committee’s first recommendation was, in part, for governments to “inventory critical transportation infrastructure in light of climate change projections to determine whether, when, and where projected climate changes … might be consequential” (p. 192). Transportation networks are vulnerable to far more than climate change, however, and the need to assess network vulnerabilities and their consequences extends well beyond coastal areas.

The analytical tools of the geographical sciences are well suited to this task. Work by Peterson and Church (2008) provides an example of both the potential and the current limitations of such research. Using rail network data from Oak Ridge National Laboratories and freight data from the Bureau of Transportation Statistics,2 they developed a rail routing model to assess the loss of a rail bridge. Their analysis showed that, for all traffic going to and from Washington state that used the Sandpoint Bridge, the detours—upon the loss of the bridge—averaged 330 miles. Impedances increased as well, indicating that the selected detour routes were not ideal. Because the national rail dataset lacks data on track capacity, this study was not able to take this important variable into account. Because some routes are already operating at capacity, some freight might not be transported or trains could be forced to take even longer routes if the Sandpoint Bridge became impassable.

SUMMARY

Understanding how and why mobility and mobility consequences vary systematically from place to place will be crucial for predicting the range of likely economic, environmental, social, and political impacts of increasing mobility and altered mobility choices in the coming decades. Geographical scientists from several disciplines, including geography, civil engineering, sociology, economics, and political science, are well positioned to take up these questions.



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