Yer right about the treacherous part. I've been riding to work on occasion this winter, and I take the back roads home. The only way I can tell if I'm on mud vs. mud-covered ice pack is the almost uncontrollable wheel spin experienced on the ice pack.
This tag is pretty easy to get to: not far off the pavement at all! See picture. You have found it when you head south out of the east side of Ann Arbor on a road that crosses over I-94 but has no access ramps. Happy hunting.