A Chicago Transit Authority 2006 New Flyer bus, modified as a clean air hybrid bus. Chicago Illinois USA. May 2011., a photo by Eddie from Chicago on Flickr.
I took this photo recently during evening rush hour traffic.
The Chicago Transit Authority and many other transit agencies in North America, have done a lot in recent times to cut down on harmful diesel exhaust emmisions, and help keep the outside air clean.
CTA has a number of buses today, modified for the task of maintaining clean air with low emmisions. They carry the advertisement "Clean Air Hybrid Bus", along the roof line of the buses.