In La Jolla, CA, there’s a certain beach where seals like to hang out. There are signs with a picture of a vicious seal biting someone’s finger off, and a caption that says, “WARNING: SEALS BITE”
Maybe you can answer a question for me, Iceland. In a book compiled by Hamish Hamilton and titled Signs of the Times, there is a sign "near the old bomber base at Hethel, that says “NCC SALT DUMP”
Attached to it is a sign reading “NCC HETHEL CHIP DUMP”
Hamilton’s caption says that the sign “confirms the legends about what servicemen used to eat, and suggests that the end of the war took the quartermaster by surprise.”
I don’t quite understand this; for one thing, I don’t know what “NCC” stands for. 
Here are two that I can think of from various places in Colorado.
On a 2 lane highway running between two towns was a sign that said:
“Test area, please turn on headlights next 15 miles.”
WHHAAAA??
Then, after the 15 miles was up it would say:
“Leaving test area. Don’t forget to turn off your headlights” or something similar.
I’d drive by and, being the dutiful motorist that I am, I’d flip on my lights even though it was bright daylight outside. To this day I have no idea why that sign was there. I used to imagine all sorts of senarios where it was like some kind of weird government psychological study to see how many people would do something completely idiotic just cause a sign told them too…
The second one is a series of signs you’ll see as you are heading down I-70 out of the mountains into Denver. The signs are meant for truckers who have a hard time both going up and coming down this part of the highway cause it is STEEP.
They say things like:
“Truckers! Don’t be fooled! Steep curving grade next 7 miles” or “Truckers! You are not down yet! Steep grades next 9 miles.”
They always make me smile… something about the image of this trucker hunched over the wheel, sweat dripping off his brow, as he tries to navigate his huge rig down the scary mountain road wondering what’ll give first, his brakes or his stamina. And along the way, cheerful roadsigns to remind you how screwed you are.
Actually, that is where I cross the road after a ‘sleepless’ night.
No, I haven’t crossed for a while.
This, in any of its myriad incarnations, is one of my favorite signs.
The other is “Welcome to our ool. Notice there is no ‘p’ in ‘pool.’ Please keep it that way.”
Along 1 (I think) in Cali, just South of Monterey, windy road sign, with caption “Next 81 miles.” That made us sweat in our little chevey rental car.
I went up to the Sunday beach after the worst of Hurricane Jeanne had passed. In the insane surf was a street sign being lashed by the waves. It said " Evacuation Route" with a stylized drawing of a hurricane underneath it.
On two-lane highways, especially those with long straight parts, some people tend to pass other cars. Some people are unusually impatient and/or bad judges of timing and/or distance, and will pull out to pass when there is a car coming toward them and they don’t really have the time/distance to complete the pass. Now, if you are in that oncoming car and you look down the road and see a vehicle in your lane, it may not be obvious until it’s too late whether you’re looking at the back end of a car traveling in your same direction or the front end of a car coming toward you. Unless, of course, all cars have their headlights on. Then, even in bright daylight, it’s obvious when the car you see ahead is actually facing you.
The “Test Area” part, I assume, means that the highway department is doing a study to see if the use of headlights really causes a reduction in accidents.
My favorite: “This sign has sharp edges”: http://carcino.gen.nz/images/index.php/5922d576/1edba836
Hmmm…undead thread.
On a trip to England, we drove past signs that said “Tank Shooting Range” or something like that, with a picture of a tank on it. That was a little surreal.
In a commercial, drive-through animal park in South Africa:
Please keep your vehicle moving, as lions like to bite tyres.
Years ago there was a sign on Manly Golf Course that merely said “Do Not Lean On This Sign”.
I knew I shouldn’t have driven the Toyota!
And Australia takes another world record - 14 lamps in a single traffic signal!
But I think my favourite is the simple declaration at the entrance to one of the first freeways in Sydney: “GO BACK! YOU ARE GOING THE WRONG WAY!”. Preach it, brother.
I think New Mexico is still the only state with existential warning signs - they read, “Gusty Winds May Exist.”
I’ve always liked KOALAS CROSS HERE AT NIGHT
In the optics lab here, we have a red light over the door and a sheet protector taped to the wall under it so that we may display paper signs describing what the red light means when illuminated.
It varies. One day it could be UV hazard, another day intense laser radiation, or an admonition NOT TO OPEN THE DAMN DOOR because we mustn’t let any light in.
So, once, during a time when the light wasn’t in use… as in there was no hazard… I displayed the following sign:
WARNING: DO NOT LOOK AT LASER WITH REMAINING EYE
It did a pretty good job of demonstrating who reads the sign and who doesn’t.
I like the signs on the highway that say USE EXTREME CAUTION. I imagine that using “extreme” caution means pulling over to the side of the road. Regular caution will not suffice–it must be extreme.
I’m really impressed, C., you Aussies have taught koalas how to read! 
No, they’re just Very Angry.
Now don’t tell me they’re still doing those Qantas commercials! 