I like this one: North Carolina words their signs as "Reduce Speed Ahead" instead of the grammatically-incorrect "Reduced Speed Ahead." The latter is incorrect because it's the speed limit that is reduced and you are telling the driver to reduce his speed. (These signs are starting to disappear in favor of the new and better signs that tell you what the reduced limit will be.) I've only seen "Reduce Speed Ahead" in one place outside North Carolina—the westbound Dulles Toll Road in Virginia as you approach the main toll plaza in Tysons. I always wondered why VDOT used the different wording in that one spot.
I don't view "Reduced Speed Ahead" as grammatically incorrect. It is stating a fact, and the black-on-white rectangular format is used for informatory as well as regulatory sign messages. "Reduce Speed Ahead" is an imperative message.
North Carolina's BGSs often use the word "DOWNTOWN" in all caps underneath a city's name. I can't say as I've seen this style in most other states; more typically, I've just seen "Downtown [city name]."
Michigan also uses "Downtown" in all caps. The difference between the two states, AIUI, is that Michigan DOT uses the caps letter size, while NCDOT uses the "small caps" treatment (height of "Downtown" in all caps is equal to lowercase loop height of primary destination legend).
And speaking of Michigan, the divider bar underneath cardinal direction words (informal roadgeek term: "underlining") is a feature unique to that state, so far as I know, that has now almost completely disappeared.