Denver is testing a program that monitors GHG emissions. Not much tech info there, so I'll just summarize: *Something* is being fitted on vehicles which sends the data wirelessly to a web site. There, folks can monitor their progress as they change their driving habits. That's it. Just like we do with SG's or tank to tank measurements. Since they don't mention *how* they are measuring GHG emissions, I'll guess they are measuring fuel consumption and calculating emmissions from that.
"The hope is that once drivers see the cumulative effects of their gas-guzzling patterns—like idling, speeding, fast stops and hard breaking—they will be more likely to make changes and reduce their fuel consumption, thereby reducing GHG emissions."
http://www.redherring.com/Home/23929