This is a green sticker for a German low emission zone. Such zones were decreed this March in several cities of Germany. Cars that do not have a sticker are not allowed in low emission zones, i.e. if you're caught without you get fined. There are green (da cleanest), yellow (not so good) and red (barely passable, mostly diesel engines) stickers. The sense or nonsense of this government measure is subject to heated discussion. Owners of older cars – who tend to be the less affluent – are pissed because they are forced to either invest in a particle filter (which is not even available for some makes) or to get another car. While their old polluter will continue polluting somewhere else where there are no low emission zones – most likely in an East European country. While the main polluter still remains the industry, etc., etc.
Another and the final contribution to Anna Carson's Project Green, which comes to an end with take nine.