Harmful emissions from U.S. vehicles dropped to a record low in February, on an individual driver basis, the University of Michigan Transportation Research Institute reported Monday.

The average buyer's new vehicle produced 23 percent fewer emissions in December 2013 than in October 2007, the institute said.

The institute's "eco-driving index" measures both vehicle fuel economy and the distance driven. Improving fuel efficiency and a decline in miles driven have helped reduce greenhouse-gas emissions, the report said.

- Paul Nussbaum