Bridging Data Gaps
"When the I-35W bridge collapsed a year ago in Minneapolis, federal officials immediately requested an emergency inspection of every similar bridge in the nation. There was just one problem: No one knew how many there were.
At first, officials thought there were 756 steel deck truss bridges like the one that fell. That's how many they found in the official federal database of bridges, the National Bridge Inventory, which gets its records from the states. Then state engineers found 32 more to add to the list.
By the time the survey was finished, the count of bridges of the same type as the Minneapolis span was down to 479, or 277 fewer than initially reported."