This is the only thing I could find, and the goods and services registered is wine, not clothing.
But it is in Michigan, though!
Trademark Electronic Search System (TESS)
EDIT:
Nevermind, I did find it:
http://tess2.uspto.gov/bin/showfield...003:iukfbe.4.1
The trademark is registered, but they still don't have a trademark on the "concept" of using a highway sign on clothing.
The following is not legal advice....I can see how they would want to protect their trademark, but the problem is that they are using something that is a public item as their brand. Can they sue, maybe, but I think it would be extremely hard for them to win.
The state of Michigan had those highway signs designed for a purpose. It would be interesting to see what they would do if the state decided to sell shirts with the highway signs.
Now that would be an interesting case!