traffic2Which American city has the worst traffic? If you said New York or Los Angeles, guess again. A new survey by TomTom, which is based on the traffic data of millions of GPS users, finds Seattle has the highest percentage of congested roadways.

The results come from TomTom’s speed profiles, an historical speed database from TomTom map business unit Tele Atlas. The profiles aggregate the actual speeds that millions of anonymous, GPS-enabled drivers have traveled over the last two years.

Cities were ranked as most to least congested according to how fast cars could travel on the street network. A city’s traffic was defined as congested if drivers could travel at only 70 percent or less of the posted speed limit, meaning on average an hour long commute included 20 minutes or more of significant delays.

According to TomTom, the numbers contained some surprises:

✓ Seattle is the city with the worst traffic in the United States, with an average of 43 percent of its roads showing heavy delays. Rounding out the top five cities after Seattle are Los Angeles (38 percent), Chicago (37 percent), San Francisco (35 percent); and New York (31 percent).

✓ California’s Bay Area has the most cities in the top 30, with San Francisco, San Jose (29 percent) and Oakland (28 percent).

✓ The most congested corridor is between Baltimore and Washington, D.C. About 36 percent of that area’s roads, largely in Montgomery County, are congested.

✓ While New York has many pockets of heavy congestion, the area also contains many major arteries for alternative routes. In fact cities with fewer options for alternative roads were higher on the list.

The least congested of the top 30? Charlottesville, Va., where only 19 percent of the roads in that city are congested.

Here’s the full list:

1. Seattle (43%)
2. Los Angeles (38%)
3. Chicago (37%)
4. Montgomery County, MD (37%)
5. San Francisco (35%)
6. New York (31%)
7. Washington (30%)
8. San Jose, CA (29%)
9. Alexandria, VA 28%
10. Oakland, CA (Alameda County) (28%)
11. Atlanta (Cobb County) (27%)
12. Boston (27%)
13. Fairfax County, VA (26%)
14. Philadelphia (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 (23%)
21. Contra Costa County, CA (23%)
22. Arlington, VA (23%)
23. Orange County, CA (20%)
24. New Orleans (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%)

(Photo: Splorp/Flickr Creative Commons)