A couple of interesting links on combating CO2 in cities. These I found. They are very interesting to read and contain a lot of important information.
CO2 development Zurich
Text about a diesel ban
Also very interesting is the report from the city of Zurich "Stadtverkehr 2025" (City traffic 2025) where you can find important statistics and starting points all around the topic of traffic in the city of Zurich.
Report "Stadtverkehr 2025"