Already from March 1, 2007 onwards, vehicle restrictions in environmental green zones can be issued in cities and local districts in Germany. Therefore the respective zones were specially marked as being green zones by the city or municipality. The first environmental green zones are coming into force as of January 1, 2008 in the cities Berlin, Cologne, and Hanover. As from this date on german as well as foreign vehiceles are no longer permitted to enter these zones without the environmental badge.

more informations here: Environmental badge