A quick tip: if you are travelling in Germany, you need to avoid city centres or be aware of the environmental badge required for green zones.
Because you could get fined.
In the example given below, Bremen, you can see that, at the moment, cars with yellow or green environmental badges can enter, but that by July of this year the yellows will be excluded. Where to find out more information? Try your motoring organisation or a web site such as http://www.umwelt-plakette.de
which gives 18 languages. By clicking on the various flags I have recently discovered that the French call the badges pastilles écologiques, the Dutch refer to milieuvignetten and the Spanish call them distintivos ecológicos. Interesting that the Dutch and the Finish talk in terms of milieu or miljö. Other fascinating linguistic alleyways beckon, but to return to our subject:
Which cities are affected and which areas of them?
See the map on the above web site by clicking the cities panel on the left. The site also gives a .pdf map of the whole of Germany if you scroll down to the bottom of their home page. They sell the badges, including a 24 hour service.
Must you have a badge?
No. If you decide not to bother with it, but do want to visit the cities, then you can always park on the outskirts. It will be a good opportunity for you to use the public transport, like a local and you can talk to a few of them too.
Gute Reise! Bon voyage!