Well, no, not when it’s a command, but we do also have the wonderfully succinct “Reduce speed now”.
‘Error’ when part of a dialect? (I’ll warn you, this is a pet peeve of mine!)
Citely?
I don’t think I’ve seen ‘slowly’ on a sign.
‘Slow’ is quite common though:
http://www.pro-tectsafetysigns.com/images/productimage/761.jpg
…but it’s not common in the south, where I grew up, I don’t think. I was thinking more Northeast US or W. Pennsylvania or somewhere like that.
I actually don’t care for that particular practice. It sounds wrong to me. Of course, most folks probably think all my vowel sounds are wrong. :o
The problem with this bit (poetic as it ls) is that it modifies the wrong thing, if you’re going to call it an adverb. Sounds wrong to me, too.
No, if they had wanted that they would have used quotation marks just as you did to make it unambiguous. The slogan as it actually appeared was meant to attract attention by appearing wrong at first glance, and revealing the second meaning (your interpretation) upon further thought. Unfortunately, as we well know, the vast majority of people are incapable of thinking once, much less twice. So it simply served to provide an apparently authoritative example of an “error” being acceptable.
Personally, I also objected to the appropriation of such cultural icons as Einstein, Edison, and Earhardt for crass commercial purposes, but that’s not relevant to this discussion.
As Mangetout points out, the sign is an imperative verb (as in “slow down”), not an adverb, and is the same on both sides of the Atlantic. Have you seen UK signs that say “Slowly?”
This is becoming a real ug discussion.
Ug? Ug smash!!
No, wait, that’s not right either…
OG SMASH!
I haven’t, but I read something on this topic years ago, and it stuck to my mind as being a little strange. Next time I go to England I’ll drive slowly anyway. 
[R. Ermey] Here you are all equal worthless! [/RE]
Under 10mph on a motorway?
Jebus wept
She drove overly carefully
Hmmm, I would say “over carefully”. Which Google bears out; I get 180 hits for “overly carefully” vs. 145,000 for “over carefully”. But it seems like “overly” would be more correct.
Doing a little research, it looks like “overly” is a relatively new word, coined in 1806. Btw, Merriam-Webster says “different” has been used as an adverb since 1744 and makes note use of it being considered incorrect. Maybe that’s a distinction betweem American and British English.
MW also talks about “real” and “really”:
Be careful ( 
) with googlefights such as this. Add the word ‘egg’ and you get 8 results, all giving a whole different context for ‘over’: “…you might want to flip the egg over carefully…”
You might be right but it does look like “over” as an adverb has a much longer history (dating back to before the 12th centruy) than that “overly” which only goes back 200 years.
I’m not sure how I originally intended to phrase this but MW does not indicate that using “different” as an adverb is incorrect in anyway adn list “differently” as a synonym.
I’ll put it another way:
She was over careful
Language be changing right quick, ya feel me? Y’all gotta look real deep at what language is, and what it i’nt. Folk don’t look too close at what they sayin, they just say it. And they say it all natural-like, and that the only way it gotta be.
No, it’s an ‘error’ because the person in question done did real bad.
Verily.