German Cities And Low Emission Stickers

Berlin, Koeln and Hanover have introduced "environmental zones" to reduce fine particle emissions from traffic.

Drivers now have to display a coloured sticker on their vehicle to enter the inner city zones. The colour depends on the pollutants the vehicle emits. The cities are gradually phasing in fines of 40 euros (£29) for anyone caught driving without a sticker.

It's another in a series of examples of how the urban level can use carrots and sticks to shift behaviour.