HOUSTON – The images are hard to forget -- hundreds of vehicles could be seen plummeting into the waters of the Mississippi River in 2007 when the Interstate 35 bridge in Minneapolis collapsed.
Thirteen people died and another 145 were injured.
In the wake of the tragedy, the National Transportation Safety Board found that the bridge had a design flaw that led to the collapse.
They also found that inspectors had classified the bridge structurally deficient in previous years, which the Federal Highway Administration defines as requiring significant maintenance and repair to remain in service.
"One-hundred, fifty thousand bridges in the nation are in this category. They are deficient," said Dr. DJ Belarbi, a civil engineering professor at the University of Houston.
Local 2 Investigates analyzed the Federal Highway Administration's national inventory of bridges for 2013 and found 260 bridges in the Houston metropolitan-area listed as structurally deficient.
According to the data, the Interstate 45 northbound bridge over White Oak Bayou in north Houston was the busiest bridge in the area with the deficient rating.
[READ: List of deficient bridges in Houston]
The bridge hosts more than 124,000 vehicles per day. Exposed steel reinforcement is visible on the substructure of the bridge where concrete has fallen.
Cracks in the concrete are visible at bridges on South Braeswood Boulevard near Fondren Road and at the bridge at the Crosstimbers ramp entrance to I-45 southbound.
Both bridges have daily traffic counts in excess of 25,000 vehicles daily and are classified as structurally deficient.
The I-45 southbound exit ramp at McKinney in downtown Houston has similar issues and nearly as much traffic. Belarbi said the broken concrete itself is not an issue, but the exposure of the steel to the elements that is cause for concern.
He pointed out areas where steel was exposed at the bridge over Country Club Bayou on South Wayside Drive in Southwest Houston. The database lists its average daily traffic at 30,000 vehicles and states it was built in 1931, making it the oldest deficient bridge in the Houston area with a significant traffic count.
Surveyors were on site at that bridge this month, but a representative from the Texas Department of Transportation said they have no current activity there and did not say if there were any plans to replace it.
TxDOT is responsible for the maintenance of most of the deficient bridges found in the Local 2 analysis. A representative from the agency declined to make anyone available for an interview.
"TxDOT has been proactively inspecting bridges in Texas for nearly 40 years. Every bridge on the state system is inspected every 24 months and is safe," wrote a TxDot representative in a statement. "A structurally deficient bridge is one with routine maintenance concerns that do not pose a safety risk or one that is frequently flooded. To remain open to traffic, structurally deficient bridges are often posted with reduced weight limits that restrict the gross weight of vehicles using the bridges."
No weight restriction signs were noted at the White Oak Bayou, or Country Club Bayou Bridge, or several other bridges that are listed as structurally deficient.
"We cannot expect them to the same thing forever. Either they need to be repaired or they need to be replaced," Belarbi said in reference to the deficient bridges.
TXDot did not respond to a request to provide information regarding plans to replace and repair area bridges.
Belarbi and his team of researchers are currently studying materials to use in bridge repair and reconstruction that would not corrode. He said they are currently experimenting with carbon in lieu of steel.
"I call the corrosion of the steel reinforcement the corrosion of the structures," he said.