Malcolm Gladwell had an article a few years back about car emissions saying 5% of the cars cause 45% of the pollution and one person was quoted as advocating mobile testing. I wouldn't be surprised if this was related to that idea/research.
Here's a link to the article from his website, the sections are numbered. The one about cars is #4.
http://www.gladwell.com/2006/2006_02_13_a_murray.html