IStockPhoto 6919548 © Charles Schug

In The Graduate, Benjamin Braddock, played by Dustin Hoffman, drives his Alfa Romero across San Francisco’s Bay Bridge to reach the woman he loves. If he’d set out on that quest in early September, 2009, young love might have been thwarted. The bridge was deemed unsafe at any speed.

On September 3rd, 2009, a significant crack was discovered in the bridge, renewing concern about the safety of America’s bridges. There are over 600,000 bridges in the United States, ranging from tiny one-lane wood bridges over small streams to vast landmarks like the Brooklyn and Golden Gate. In New York City alone, according to the New York Times, there are 2,027 bridges.

The 2009 “Report Card on America’s Infrastructure,” issued by the American Society of Civil Engineers, gave a “C” to our country’s bridges. And according to reports from the Department of Transportation, 1 in 3.97 American bridges, about 152,000, are rated as deficient.

Of course, deficient is a vague term. It refers to a bridge being either structurally deficient or functionally obsolete. Structurally deficient bridges have become unsafe for certain speeds and weight due to decay, flooding, or other conditions. Functionally obsolete bridges are those that were built with insufficient width or vertical space, or have other intrinsic properties which don’t conform to modern standards. Rated at these more specific levels, 1 in 8.41 bridges are structurally deficient while 1 in 7.52 bridges are functionally obsolete. Ideally bridges in both categories would be replaced, according to the Department of Transportation. The challenge is getting the replacement – or at least repairs – in place before a tragedy occurs.

In 2007 a bridge over I- 35 in Minneapolis collapsed into the Mississippi River with over 50 vehicles on it, killing 13 people. In June 2008, a railway bridge in Cedar Rapids gave way under heavy floodwaters. To identify the most dangerous bridges, the Department of Transportation conducts inspections, noting the structural condition and age of each. It was a routine inspection, repeated every two years, which uncovered the dangerous crack in the Bay Bridge and forced its closure until repairs could be made. The bridge, which first opened in 1936, carries between 260,000 and 280,000 vehicles a day.

The odds a bridge is deficient does go up with age. For instance, no bridges built between 2003 and 2007 are deficient, while 1 in 4.1 bridges built between 1964 and 1973—the same era as the one that collapsed in Minneapolis—are either structurally deficient or functionally obsolete. The odds a bridge is deficient reach truly worrisome levels after a century. Of the approximately 14,000 bridges built before 1912 in the US, 1 in 1.55 (65%) are deficient. Recommendations for how to deal with bridge deficiencies have been made, but many worry that the current economic situation will slow or otherwise hamper the replacement of deficient bridges in America.