The German government is considering several measures for improving the quality of air in major cities, including making all inner city public transport free to use.
No German city currently offers free public transport.
The proposal foresees that the federal government would support local governments financially if they were to take it upon themselves to offer free public transport.
It specifically referenced a billion-euro programme to improve air quality in German cities, which has already been approved.
The letter further states that the federal government will support cities in efforts to introduce transport rules which would reduce emissions, for example banning heavy vehicles from city centres.