Well, that's just dumb. Why don't you talk to an engineer or two, and a couple of ironworkers, and find out what it would take to bring down that bridge with blowtorches? Even if one could do it out in the open...
Talk about dumb...you don't consider some nuts that would actually try and cut the bridge in half a serious enough threat? You probably don't think 18 guys with box-cutters are any threat either...