"Looking at the national numbers, you can see that this decline is fairly even -- there are no sharp jumps or drops, which suggests a real and sustained long-term trend toward better bridges. I asked the Department of Transportation what's behind the steady improvement. "Bridge conditions overall have relatively improved over the years but this has been the result of the strong commitment to bridge safety by FHWA [the Federal Highway Administration] and state departments of transportation," they said in a statement. "We are still not where we need to be since almost a quarter of our nation’s bridges are still in need of repair or of some type of improvement.""
I also liked this piece, Mapping America’s most dangerous bridges.