DETROIT -- Emissions from recently purchased new vehicles have fallen 14 percent over the past three and a half years, according to a University of Michigan study.
The decline underscores consumers' shift to smaller vehicles and industry efforts to reduce pollution and boost fuel economy, says Brandon Schoettle, a University of Michigan research associate who helped create the index.
The Eco-Drive Index estimates the average amount of greenhouse gases emitted monthly by U.S. drivers of new vehicles bought over the previous month.
But the index only monitors emissions from a small slice of the nation's overall vehicle population, which continues to age.
In April, the most recent month for which data are available, the index was 0.86, 14 percent lower than the base line of 1.0 in October 2007. The lower the index, the less the emissions.
The index was launched in October 2007 to coincide with the start of the 2008 model year when updated EPA fuel economy standards took effect.
The university released index results for the first time last week.
The index estimates the amount of greenhouse gases emitted based on vehicles' fuel economy and distances driven.
Skyrocketing fuel prices, higher sales of fuel-efficient vehicles and an overall decrease in driving contributed to the drop in the index, according to Schoettle, who created the index with University of Michigan Professor Michael Sivak.
Schoettle said the index marks the first time exhaust emissions have been measured in this way, adding that he hopes it can be used as a benchmark for the auto industry.
The federal government first implemented fuel economy standards to limit vehicle emissions in 1978.
The Obama administration is drafting new standards that could raise corporate average fuel economy standards to up to 62 mpg by 2025.
Response to the index from the industry has been generally encouraging since the index was unveiled in late June, Schoettle said.
The decrease in emissions shows that consumers are willing to buy more fuel efficient vehicles, he said.
"It has some positive messages for the automotive industry," Schoettle said. "People have bought, and will buy, fuel efficient vehicles."