NEW YORK (MainStreet)—The collapse of the Interstate 5 Bridge over the Skagit River 60 miles north of Seattle once again raises the lingering question of bridge safety in America. Amazingly, there were no fatalities in this latest bridge failure, but the fact remains: a large number of well-traveled bridges in America are on a "watch" list.

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In fact, one quarter of bridges in the U.S. are either structurally deficient or functionally obsolete, according to the American Society of Civil Engineers. And it's not just a rural area problem. The ASCE says that daily over two million trips are taken over deficient bridges in the nation's 102 largest metropolitan areas.

Most U.S. bridges are inspected once every 2 years, and while bridge quality has been improving over the last ten years, the average U.S. elevated span is 42 years old. The ASCE notes that more than 30% of existing bridges have exceeded their 50-year design life. While $12.8 billion is currently being spent on bridge improvement, the Federal Highway Administration estimates that to eliminate deficient bridges by 2028, the U.S. would need to invest $20.5 billion annually.

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Physical deterioration of a bridge can throw it into the "structurally deficient" category, while "functionally obsolete" bridges may be structurally sound but built to an old standard of traffic demands, such as a 1930s bridge with narrow lanes and shoulders.

The I-5 bridge was in the "functionally obsolete" category.

Though a bridge may be classified as "deficient," that does not necessarily mean it poses an immediate danger. The Bureau of Transportation statistics says, "A 'deficient' bridge, when left open to traffic, typically requires significant maintenance and repair to remain in service and eventual rehabilitation or replacement to address deficiencies. To remain in service, structurally deficient bridges are often posted with weight limits to restrict the gross weight of vehicles using the bridges to less than the maximum weight typically allowed by statute."

According to reports, an over-weight vehicle is suspected in the Interstate 5 bridge collapse in Washington State.

The worst bridges in America, according to the ASCE, are in Pennsylvania with over 24% rated as structurally deficient. Iowa and Oklahoma are runners up at 21% each. And politicos in limos beware: 77% of bridges in Washington D.C. are rated with an overall "deficient" grade, when combining the structurally deficient and functionally obsolete.

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You can find a national map of structurally deficient bridges here. By clicking on your state you can drill down to the at-risk bridges in your neighborhood.

--Written by Hal M. Bundrick