Motoring

On 1 January 2008 the cities of Berlin, Cologne and Hanover introduced low emission zones to mitigate air pollution caused by fine particles. In the near future other German cities will follow the example.
Low emission zones are identified by traffic signs and additional signs. The Ordinance on the marking of vehicles (35th BImSchV) stipulates that vehicles have to be marked with stickers (on the windscreen inside the vehicle) and lays down the criteria vehicles have to meet for the different kinds of stickers. Stickers are valid for all low emission zones in any city in Germany.
An overview of low emission zones, both existing and planned, was drawn up by the federal environment agency.
Cities furthermore provide important information about their low emission zones on their websites.
Emissions groups
Vehicles are divided into emissions groups according to their particulate emissions and they receive a sticker of the colour marking the respective emissions group:
red for emissions group 2,
yellow for emissions group 3
and green for emissions group 4.
Vehicles with stickers for emissions group 2 or above may enter the currently existing low emission zones. Stickers are only mandatory for cars driven within these zones. They are not necessary for crossing the border into Germany.
Article 6 of the Ordinance on the marking of vehicles clearly lays down which vehicles belong to which emissions group. If registration papers do not identify the European emission standard of a vehicle, the date of the vehicle's first registration is used as a basis for classification. Vehicles with diesel engines can generally reach the next higher emissions group if they are retrofitted with an emission control device such as a particulate trap. Proof of the standard achieved through retrofitting (stipulated by the Ordinance on the marking of vehicles) must be provided in form of a certification issued by the garage.

Where to obtain emissions-control windscreen stickers
Stickers can be obtained from the local licensing administration or other competent bodies designated under Länder law. Another option are bodies authorised to carry out waste gas examinations (garages, service stations with garages and inspection agencies such as Dekra and TÜV (Technical Inspection Authority) in Germany or even their offices abroad).
Stickers will be issued by any of the above-mentioned bodies upon presentation of vehicle registration papers.
It is also possible to order stickers from one of the above-mentioned bodies before driving into a low emission zone. Registration papers may be submitted (in principle copies do not need to be officially certified) via fax or e-mail (scanned document) or as copy by post. The sticker will then be sent by post. The issuing bodies charge between 5 to 10 euro for one sticker. Costs for shipped stickers, including postage, may be higher.
Exemptions are possible in accordance with Article 1 (2) and Annex 3 to Article 2 (2) of the Ordinance on the marking of vehicles. It is not permitted to enter a low emission zone without a sticker or special authorisation (fine: 40 euro). You may also obtain a sticker from your local BFG office.

Information found at: http://www.bmu.de/english/air_pollution_control/general_information/doc/40740.php