Stickers for foreign-registered vehicles
The driving bans to be introduced in German cities to curb PM (particulate matter) emission levels will
also apply to foreign-registered vehicles. Access to restricted areas will be subject to carrying a
sticker specifying the Euro emission classification. Such stickers will only be available in Germany.
Further information is provided below.

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(ADAC)

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These are the road signs displayed at the entry & exit points of the green zones.

This ridiculous regulation has already some gaps: if you have a well-maintained car that is older than 30 years, has a historical license plate or something similar to this, you can make sure the age of your car and its continous "life in trafic", than you have unlimited acces to these green areas. The same regulation meets cars are older than 20 but not already historical ones, in Germany equiped with license plates beginning with "xxx-07xx"...