Something occasionally seen in the United Kingdom (and maybe the USA, I don’t know) is the Polite notice. Usually it will simply be an instruction, such as please drive slowly, or do not enter. The signs always follow a certain pattern, they are always blue and white and they always only have a very short instruction on them. There is a very sensible reason for this, they are not meant to be read, they are merely meant to be taken in.
The word “polite” looks very similar to the word “police.” Police notices are, on the whole, taken notice of and obeyed, after all, who can afford any more points on their licence in the age of the speed camera. It is for this reason that notices which are blue and white and look like they are a police order are seen, your brain will often process it as an order and obey it without re-reading it.
However there is one flaw in this. Often if a person does notice the sign’s true wording the “screw you effect” will take over and the person will deliberately ignore the sign. For this reason these signs are designed not to draw too much attention, but sometimes something which looks ludicrous, such as “polite, slow,” will grab your attention, and lead to the accelerator being pushed to the floor.