America has a big bridge problem. There are more than a million bridges in the United States, and most were built during the great highway construction boom of the 1950s. They were designed for the relatively light panel trucks of the Eisenhower era, rather than the massive rigs of today. Meanwhile, the frequency of trips has surged. In 1950, trucks hauled 200 billion ton-miles of intercity commercial freight; by 2002, that number increased to 1.44 trillion ton-miles, or more than 600 percent.