The Ruhr is showing its environmentally conscious side:
Starting in 2012, to improve the air in the region, the entire area is being considered an integrated environmental zone.

Only the inner cities had been covered previously.
As of January 1, 2012, the three-step plan envisions a mandatory environmental sticker (red, yellow and green) on all vehicle windshields.

A year later, you’ll only be able to travel from Duisburg to Dortmund with yellow and green stickers.
And starting July 1, 2014, the air will be finally pure and only cars with a green sticker can travel the roads.

Exceptions naturally prove the rule. There are special regulations for trucks, and the inter-city autobahns are excepted, as are boats on the Rhine and the Ruhr rivers.