OK, that's
definitely a New Hampshire Adopt-a-Highway sign as seen
here
The Illinois Adopt-Highway signs look very different as seen
here
And even though the jalopnik.com site states that it's a fake didn't originally state it as such, which is how rumors get started.
What I find highly suspect is that all references to this story point back to this single columnist for the Chicago tribune, who supposedly has a "real" photograph (but no evidence of one). This in contrast to the KKK story which has multiple sources.