PORTLAND -- An annual study of traffic flow worldwide ranked Seattle 4th and Portland 8th for the worst city in the U.S. for congestion on arterial roads and highways.

The TomTom Traffic Index tracked time delays for drivers, peak traffic hours and congestion levels in metropolitan areas.

The research indicated that Portland drivers were most likely to get caught in backups during the morning commute on Thursdays and Fridays.

It also found that the worst nights for rush hour congestion were Monday and Thursday evenings.

Los Angeles was listed as the most congested city in the U.S.

Seattle was ranked as number 4 in the U.S. and 7 in the world. Portland was ranked as 13th worst in the world and Rio DeJaneiro was number 1.

The TomTom Index is used as a tool for city planners evaluating areas in need of upgrades or changes to improve traffic flow.

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