Could the spinning disc be something as simple as a device to ensure birds do not alight on the box?
It appears to be a type of roadside pollution monitor. Here is alink with a picture of a similar looking device.
So I finally had a chance to pull over and see what this thing was. Driving by today it looked as if someone had put down fresh gravel on the side of the road. This meant that I had a place to pull over that wasn’t part of a muddy field. Anyway, here are a few links that discuss what this contraption is. I know all of this because I found the label on the side.
http://www.stusteel.com/rectifiers-IRT.htm
If you ever see something like this, now you know what it is! Thanks everyone!
Now we know the rest of the story. - Paul Harvey
I’m glad you posted the results. See people read your threads.
We’ve still no explanation for the 1" thick spinning disk on top.
I suppose it could be camouflage.
Ionic disapator or a moving target for birds. 
Yeah I’m still not sure what the disk is for. It’s really nothing special, just appears to be a metal disk attached to a spinning metal rod. It has a little bit of gray paint on it to match the housing. I didn’t see any rectifiers for sale that had that option either. I guess that one will have to remain a mystery for a bit longer. I might be able to get another look at it tomorrow but I’m not really sure what more I can figure out just by looking at it. I also don’t want to caught by the police scoping it out and photographing it. In this day and age that might be a bit suspicious!
Or there’s a guy with a perverse sense of humor who said to himself, let’s add a spinny thing to the next traffic light control box and see who notices. Mebbe he’s a SDMB member who’s lifting a cold beer right now while laughing his buns off at this thread. 
Could it be a 1920s style death ray?
Nah. Spinny things weren’t commonly added to death rays until the late 1940’s, shortly before tail fins became the sought after option. 
Do you think you could see the disc from a low/slow flying plane?
Perhaps if it does not spin it is not working?
Justa WAG.
I can’t imagine it would be very effective when its the same color gray as the enclosure.
I sent an email about spinning disks/cathodic protection to the media/community relations person at a large pipeline company, and hopefully will get an answer.
I just gots to know!