Japan has a number of car stickers that signify different things. These include a butterfly and what looks like a blue shamrock. The stickers are for the safety of the driver and all those on the road. One such sticker looks like a tear drop and is half yellow and half red. This marks means that a driver is over 75 years old. All drivers of this age must display these stickers on their car to notify other drivers of their elderly status.
Similarly, for the first year after they get their license, young drivers must display a sticker that is half dark green and half light green. This warns those around them that they are not very skilled.
The blue shamrock mentioned earlier means that a driver is handicapped. And the butterfly means that the driver is hearing impaired. It is unclear how these symbols correlate with each of these conditions though.