As a civil engineer for over 40 years, I can think of dozens of relatively simple ways to cause major long-lasting chaos over a wide area with coordinated attacks. I won't list them for obvious reasons, but taking out hundreds of bridges and overcrossings in a large metropolitan area is definitely one of them. And "Law enforcement" acts oblivious to it.
As a civil engineer, do you really believe this would be "relatively simple" ?
Even if we contracted this job out to the top demo crews in the country it would a very tough task. Blowing up a bridge, let alone "hundreds" of them isn't quite that easy. Especially if you are trying to do it covertly. I do agree taking out maybe one bridge like the Wilson Bridge or the Golden Gate would cause probs for a while and we should be aware of this, but when you see a girl on a foot bridge in Columbia Md. looking at the cars and somehow assume she's Al Queda on a mission is quite a leap. This is a VERY populated area.