At either end of Route 50, there is a highway sign that tells you how far the opposite end is. I carved a pair of letterboxing stamps for these signs. Here is a picture of me with the one in California:
It was kind of an awkward place for a letterbox, but I figure if someone is already getting out to get their picture taken, I am not encouraging dangerous highway behavior, maybe. Each letterbox is in a camo pouch. You can see it's the stamp I used for the Transamerica 2012 postcards.
Here is a photo of how good I hid it. The freeway shoulder area was very barren, so I stuck it down alongside one of the sign posts.