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In Brown County on the northern border of South Dakota, for example, four bridges fall into both safety categories, each of them a century or more old. One has already been closed. The other three are dying of old age, said county highway superintendent Jan Weismantel, but they should be safe as long as drivers don't exceed restrictions placed on the size of trucks and other vehicles — 3 tons for one bridge, 5 tons for the other two. Those restrictions effectively limit bridge traffic to a car or light pickup truck.
All bets are off if someone drives a heavy truck or farm machinery across such a bridge, she said.
"Who's to say that somebody's not taking something that weighs more than 6,000 pounds across a 3-ton max bridge?" Weismantel said. "It's kind of a gamble. It's also very dangerous. Somebody could get killed."
James White, a 33-year-old independent trucker, said he worries about the condition of some of the bridges he drives his 18-wheeler across in the mountainous terrain of eastern Tennessee.
"There are some that I go across with a 90,000-pound load, and I'm thinking, 'If this bridge don't hold me, I'm gone,'" he said while refueling at a truck stop in East Nashville.
Michigan bridges in both categories include some that don't see much traffic. Three are over the Whitefish River in Alger County in the Upper Peninsula. They are steel truss bridges, with a surface made from timbers, and probably need to be replaced.
"I applied for federal bridge funds for five consecutive years. Each year I'm denied," said Bob Lindbeck, engineer and manager of the Alger County Road Commission.
He said the condition of the bridges forces him to post weight restrictions, which in turn discourage people from living in the area year-round because snow plows and emergency vehicles can't cross the bridges.
In eastern Kansas, Miami County engineer J.R. McMahon said many of his county's bridges were built nearly a century ago, when farm equipment wasn't much bigger than a modern full-size sport utility vehicle. As the bridges get older, they become "functionally obsolete" because they are too narrow to handle agricultural traffic, he said.
"It's an issue of priority for the counties," said Norman Bowers, local road engineer for the Kansas Association of Counties. "Do you spend money to replace a bridge when only 10 people a day use it?"
Pennsylvania has whittled down its backlog of structurally deficient bridges but still has many more to go. Some are among the busiest in the state, including the 85-year-old Liberty Bridge, spanning the Monongahela River from downtown Pittsburgh to its south side, and an I-95 span in Philadelphia's lower northeast section along the Delaware River. And an estimated 300 bridges are in position to move onto the state's structurally deficient list every year if no maintenance is done.
Many Pennsylvania lawmakers have long sought to boost transportation funding, in part to address crumbling bridges. But this year's proposals, including Gov. Tom Corbett's $1.8 billion plan, stalled amid fights over details.
New construction techniques are being employed in some states to significantly reduce the time it takes to replace deficient bridges. In some cases, most of the superstructure is being constructed off-site and then moved into place when piers and abutments are ready. States are also packaging bridge design and construction into a single contract, saving more time.
Massachusetts' "Fast 14" project replaced 14 bridges on Route 93 near Boston in 10 weekends. State officials estimated the work would normally take at least four years, but the time was dramatically reduced by relying heavily on prefabricated parts.
Congressional interest in fixing bridges rose after the I-35W collapse in Minneapolis, but efforts to add billions of federal dollars specifically for repair and replacement of deteriorating bridges foundered. A sweeping transportation law enacted last year eliminated a dedicated bridge fund that had been around for more than three decades. State transportation officials had complained the fund's requirements were too restrictive. Now, bridge repairs or replacements must compete with other types of highway projects for federal aid.