They are actually used in Canada as warnings.
If a car is used by someone who is learning to drive, it must have an "L" sticker and the learner has to drive with a licensed driver.
"N" signifies a new driver who has had their license for less than a year.
The "C"?
Ask any driver in British Columbia who are the worst drivers, they will answer "Chinese" (immigrants)
At the same time as C, they came out with "P" for pensioner. You guessed it...