Well, as part of the new Federal transportation bill, MAP-21, the U.S. Department of Transportation is being charged with defining "congestion," and that could be a big job. That's because there is apparently a lot riding on how DOT decides to measure congestion. It's currently measured by how much your commute is slowed down by traffic. So if it's free flowing, guess what? No congestion. And while that's a good thing for air quality, communities with overbuilt road capacity are being rewarded for not having congestion while they should probably be considering transit and alternative transportation options instead of building more roads.
Some folks now say the question is not "What is congestion" but if congestion is even the right thing to be measuring? Streetblog tackles that question here.