Maybe they are permanent warnings. Found another on-line comment and photo
http://www.albertelli.com/photoarchive/trip/Day2/tn/IMG_0948.JPG.html.
Does make you wonder how dumb people are.Then I found this:
Signs In America...
“Do not drive into smoke”
This is a sign you will see if you drive through Oklahoma. Why would they post this sign along their highways? I have developed several theories.
Theory #1
People in that region refer to “fog” as “smoke”. Seems reasonable enough. Driving into thick fog could be dangerous.
Theory #2
Oklahoma is made up of many Indian reservations. Indians, according to ancient lore, communicate via a medium known as “smoke signals”. If you were to drive through these signals you might jumble them up, causing the Indains to think people are sending them gibberish messages. That's just plain rude.
Theory #3
Where there’s smoke, there’s fire. And you wouldn’t drive into fire, would you?
Then, we looked on the internet (which knows everything). This is the explanation we found: These signs stem from a huge pileup that occurred when a farmer burned his fields, causing masses of smoke to linger upon the highway, and ultimately cause over 40 cars to careen into one another.
Other signs:
“Warning: Regional ice may be present”
Hmmm, is that a special ice? Ice with an accent? I guess in Arizona it is. Who knew it got that cold in Arizona. I hate the cold.
“Gusty winds may exist”
Yes, and so might space aliens. A lot of things may exist. That just sounds funny to me. What's wrong with "Danger, Stong Winds"?