TomTom's taking traffic congestion to the future, aggregating speed data from its in-car navigation systems to pinpoint precisely which cities have the worst traffic congestion. Here's their scientifically-derived 20 most traffic-clogged cities. The results may surprise some.

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TomTom collected data from its Tele Atlas business system GPS data to determine what streets were "congested." In order to qualify a driver has to travel at only 70% or less than the posted speed limits. And while cities like Los Angeles and New York make the top five, it's actually Seattle, Washington at the top of the pile with 43% of the roads considered congested. Full details in the gallery.

Rank: 20th
City: Houston, Texas
% Of Roads Congested: 23%
Population: 2.24 million

Rank: 19th
City: Portland, Oregon
% Of Roads Congested: 23%
Population: 557,706

Rank: 18th
City: San Juan, Puerto Rico
% Of Roads Congested: 24%
Population: 422,655

Rank: 17th
City: Long Island, New York
% Of Roads Congested: 24%
Population: 7.45 million

Rank: 16th
City: Phoenix, Arizona
% Of Roads Congested:
Population: 1.57 million

Rank: 15th
City: Austin, Texas
% Of Roads Congested: 25%
Population: 757,688

Rank: 14th
City: Philadelphia, Pennsylvania
% Of Roads Congested: 25%
Population: 1.54 million

Rank: 13th
City: Fairfax County, Virginia
% Of Roads Congested: 26%
Population: 1.02 million

Rank: 12th
City: Boston, Massachusetts
% Of Roads Congested: 27%
Population: 620,535

Rank: 11th
City: Atlanta, Georgia
% Of Roads Congested: 27%
Population: 537,958

Rank: 10th
City: Oakland, California
% Of Roads Congested: 28%
Population: 645,345

Rank: 9th
City: Alexandria, Virginia
% Of Roads Congested: 28%
Population: 140,024

Rank: 8th
City: San Jose, California
% Of Roads Congested: 29%
Population: 948,279

Rank: 7th
City: Washington, D.C.
% Of Roads Congested: 30%
Population: 591,833

Rank: 6th
City: New York, New York
% Of Roads Congested: 31%
Population: 8.37 million

Rank: 5th
City: San Francisco, California
% Of Roads Congested: 35%
Population:

Rank: 4th
City: Montgomery County, Maryland
% Of Roads Congested: 37%
Population: 808,976

Rank: 3rd
City: Chicago, Illinois
% Of Roads Congested: 37%
Population: 2.85 million

Rank: 2nd
City: Los Angeles, California
% Of Roads Congested: 38%
Population: 3.83 million

Rank: 1st
City: Seattle, Washington
% Of Roads Congested: 43%
Population: 602,000