Vehicles account for about 30 percent of Denver's greenhouse-gas emissions. That's one reason the city has agreed to participate in a new public-private initiative, Driving Change. Sponsored by EnCana Oil and Gas, the pilot program seeks to demonstrate once and for all that there is a correlation between driving behaviors and CO2 emissions -- and, along the way, persuade drivers to change their aggressive-driving ways. Starting today, devices in 400 cars -- 200 driven by city and county employees and 200 by volunteers -- will begin tracking how driving patterns such as speeding, idling and fast stopping affect fuel consumption and greenhouse-gas emissions. The devices wirelessly transmit the data to a secure Web site where drivers can see how much greenhouse gas emissions their vehicles generate. The site suggests strategies tailored to each driver's habits to improve their behavior and reduce their emissions. The primary goal of the pilot is to produce up to a 20 percent reduction in CO2 from each pilot vehicle.