I’ve commented before about a dilemma produced by fuel-efficient cars: they emit less per mile, but also make it less expensive to drive… creating a countervailing incentive to drive more than you might have in a gas guzzler.
Therefore I was happy to discover that the University of Michigan has been tracking and amalgamating the greenhouse gas emissions effects of both the emissions profile of new cars for sale, and the average number of miles driven on a monthly basis. They call the resulting trendline the “Eco-Driving Index.” They’ve been tracking it since 2007, when the EPA revamped its new-car fuel economy calculation methods.
Here’s what the trend looks like:
All the data is normalized to 2007, so if we were driving exactly the same distance, and new cars emitted exactly the same per mile today as in 2007, the index would be “1.00.”
As you can see, both “miles driven” and “fuel used per distance driven” have dropped, albeit not consistently. Still, drivers who bought a new car in May 2011 would, on average, emit 16% fewer greenhouse gases than drivers who bought new cars in October 2007.
My two conclusions: First, this is great. I’m glad to see that we haven’t gobbled up all our efficiencies by increasing our driving. Second, it’s clear that the bulk of the improvement comes from more efficient cars, not driving less. That’s the lever we have to continually adjust downward at a policy level.
You can find more details on the calculation method here.