TomTom and TeleAtlas use their crowd sourced contributors to plot worst congestion in the United States.
Congestion is considered traffic that is 70% or less of the posted speed limit.
TeleAtlas has an audio podcast on this here and a full press release here.

Rank CITY/REGION % of congested roadways
1 Seattle, WA 43%
2 Los Angeles, CA 38%
3 Chicago, IL 37%
4 Montgomery County, MD 37%
5 San Francisco, CA 35%
6 New York, NY 31%
7 Washington, DC 30%
8 San Jose, CA 29%
9 Alexandria, VA 28%
10 Oakland, CA (Alameda County) 28% (read more for #20-30)

11 Atlanta, GA (Cobb County) 27%
12 Boston, MA 27%
13 Fairfax County, VA 26%
14 Philadelphia, PA 25%
15 Austin, TX 25%
16 Phoenix, AZ 25%
17 Long Island, NY (Nassau County) 24%
18 San Juan, Puerto Rico 24%
19 Portland, OR 23%
20 Houston, TX 23%
21 Contra Costa County, CA 23%
22 Arlington, VA 23%
23 Orange County, CA 20%
24 New Orleans, LA 20%
25 Norfolk County, MA 20%
26 Denver, CO 20%
27 Morris County, NJ 20%
28 Miami-Dade, FL 20%
29 Dallas, TX 20%
30 Charlottesville, VA 19%