http://www.flickr.com/photos/vanderlin/358744332/in/photostream/
http://wwwimage.cbsnews.com/images/2007/01/31/image2419978g.jpg
The LEDs are transparent, no C4 in them. The circuit board /panel the LEDs are mounted on is very thin. No C4 in it.
The batteries are visible, I guess you could argue that the bomb material is in fake Duracell batteries, but then you'd have to explain what is powering all those LEDs, and these have been in place for weeks. A couple of them could theoretically be dummy cells with explosives inside. It wouldn't be enough to do much to a bridge pylon, especially since the charge wouldn't be placed or shaped properly for that effect.
If someone wanted to make a bomb, why would they rely on small fake batteries rather than an enclosed area at the back of the sign that wouldn't look suspicious, but could contain a significant amount of explosive.
Why would you be suspicious of this being a bomb?
I could hold enough C4 in the palm of my hand to blow the engine of your car up through the roof of your garage.
I doubt it. High explosives aren't good a moving mass, and my big block Chevy engine isn't exactly light.
High explosives are good at tearing things apart, but you'd want to use a lower velocity explosive to send my engine flying.
You could probably send parts of my engine through the roof, but if you want to send the whole engine, you've got the wrong type of explosive, and you'd need to direct the force properly.
I don't have any personal experience with C4, but I've helped a friend set a 2 pound KinePac charge. I was pretty impressed with what a high explosive can do, but I'm pretty skeptical that it could send my engine through the roof of my garage. That's not what it's good at doing. It creates an intense impulse of pressure which destroys things well, but doesn't move mass well.
Holy Cow. You have to be kidding me that all that panic was over this bit of art. What insanity!