This is the standard frost warning sign we see in Australia. 
I don’t know if it’s a universal warning, but I can see that it’s clear.
On holidays recently I saw a different version. No image, just words (and no photograph either, sorry):
Reduce speed when frosty
Perhaps it was a warning about the driver, not the road surface. Imagine if the drove home from holiday with all seven hours chilled by my frosty irritation at some tiny offence. Awful!
So, in the interests of family and driving-partner harmony, I have a series of suggested road signs that alert us to the dangers inside the car.
Stop grunting when grumpy
Don’t blame others when lost
Avoid trans fats when stressed
Indicate when carsick
Count to 10 upon hearing ‘Are we there yet?’
Perhaps these are not the most needed signs. Your experience is essential – what is needed beside our roads? Put your ideas in the comments.
And if you feel like designing a proper sign based on these (or other) ideas, I’d love to see it. Post the link or send it to me to display a properly good-looking image.