ANN ARBOR—A new national index by the University of Michigan shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 14 percent since late 2007.

The U-M Eco-Driving Index estimates the average monthly amount of greenhouse gases produced by an individual U.S. driver who purchased a new vehicle that month.

The EDI for April 2011, which is the latest month for which data is available, stands at 0.86, compared to the baseline 1.0 in October 2007, the nominal start of the 2008 model year and the first for which the Environmental Protection Agency started using the current fuel-economy rating system.

“The amount of greenhouse gases emitted when using internal-combustion engines depends on the amount of fuel used,” said Michael Sivak, research professor at the U-M Transportation Research Institute. “The EDI estimates the amount of fuel used (and thus the amount of greenhouse gases emitted) by taking into account two primary variables—the fuel economy of the vehicle and the distance driven.”

Monthly values index courtesy of UMTRISivak and UMTRI colleague Brandon Schoettle compute the monthly EDI by cross-multiplying the average amount of fuel used per distance driven by newly purchased vehicles (EDIf) and the distance driven per individual (EDId). The lower the value of the EDI, the smaller the environmental impact.

EDIf estimates the relative amount of fuel needed to drive a fixed distance. It is calculated as an inverse of the sales-weighted, average fuel economy of purchased new vehicles for each individual month. In turn, the average fuel economy (in mpg) is derived from the monthly sales figures of individual models and the EPA fuel-economy ratings for the respective models.

EDId provides information about the relative amount of driving per licensed driver. It starts with the estimates of the total distance driven in the United States each month as issued by the Federal Highway Administration. The researchers then adjust these raw distances to take into account the seasonal variations in driving, the varying number of days in a month, the continuously increasing number of drivers, and the so-called rebound effect (the increased amount of driving as a consequence of improved fuel economy of the new vehicle).

Related Links:

• Eco-Driving Index
• Sustainable Worldwide Transportation