Sometimes a person just gets lucky. I don't think I have ever seen a more perfect rainbow (the rainbow, not the photograph. It was amazing to see!). Any other lens and I would have missed it :)
Thanks all. I came within a couple of minutes of missing the entire thing. It lasted at least 10 minutes. I drove for almost all of that time trying to find a good place to pull over. Finally I just parked in the ditch, did a qucik lens change, and started shooting. I probably hadn't been gone from where I shot it for more than 2 minutes and it just vanished as quickly as it came. Everything had to be just right for this to happen. The sun was at the right altitude, my location was right, and it was raining in the right place. :)