Found this one this afternoon after locating two geocaches in the area. I was pretty sure which bridge I was looking for, but there wasn't a great spot to park, and I had to drive around for a while. Finally I found a suitable parking area up the road a bit, and I ran down to the bridge. Well, this one wasn't so easy! I knew where to look, but that area was covered with a whole lot of gravel and dirt. This was a very tricky spot in which to balance, and with the traffic whizzing by overhead it was somewhat unnerving. Still, I felt comfortable enough to take the time to dig out the mark—it was under about an inch of gravel and, I discovered as I dug away at it, something black and greasy and filthy. Coordinates were not perfect, but with the excellent description they weren't really needed. I managed to take a few photos before sliding down the hill!
Recovered in good condition as described. Handheld GPS coordinates for the bench mark are 40 50 59.6(N) 075 36 48.2(W).