There are almost 4 million vehicles registered in the D.C. metro region, up 4 percent since 2008. For years, the addition of more cars hasn't dramatically affected emissions, because newer, cleaner cars were hitting the roads. With the tough economy tightening budgets, that trend may well be over.
"When the recession hit, we started seeing people defer the purchase of new cars. The average age of the fleet now is older than it was three years ago," said Ron Kirby, head of Transportation Planning for the region.
The fleet is about a year older, on average, which means a lot of people are still using older cars and SUVs with dirtier exhaust than newer vehicles
"That means air quality isn't as good as it would've been," Kirby said.
Another way of thinking about it is that air quality isn't getting better as fast as it used to. That's important because the region doesn't meet certain federal air quality standards. The recession also hit the purchase of hybrids hard, but they've since come back faster than at any other time in history.
And while SUV's have been falling in popularity in the region for some time, they took a big hit during the recession, probably, Kirby said, due to the Cash for Clunkers program.