Many of America’s largest cities, including Manhattan, are paradoxically located in geographically isolated areas of the country. Although one in 38 Americans lives in New York City, much of the city is bound by water, making it only marginally connected to the mainland. Consequently, in the event of a major disaster, such as a grid outage, terrorist attack, or a national pandemic, the city’s eight million residents will be dependent on a few bridges for safe passage.