Low Emission Zones.

[IanIanIanIan]

In Europe there are some 'Low Emission Zones'

http://lowemissionzones.eu/

I applied for the German LEZ sticker, which covers all of Germany, and
have just received it after four weeks including Christmas. It cost €5
from the Cologne authorities and is valid for the life of the car.

[johalareewi]

Cheers. Handy overview page here

http://lowemissionzones.eu/overview-of-lezs

[Grumpy Cabbie]

It appears the interpretation of what counts as 'low emission'.

Some of those zones ban cars that don't comply with Euro 2! lol, that could hardly be classed as low emission when the NOx levels of Euro 3 was about 500 mg/km compared to the Prius at 0.06 mg/km.

I guess it means you wouldn't be able to take your traction engine in, but most other vehicles would be fine.