Seattle's traffic delays worsened enough during the first quarter of 2012 to make it the fourth-most-congested city in North America, says a report issued Tuesday.

Basically, when ordinary slowdowns were combined with the two-day January snowfall, it was enough to propel Seattle near the top. Jan. 19 was the slowest traffic day of the quarter here, while some cities with mild winters improved in the stats, said Nick Cohn, head of congestion research for the Tom Tom traffic-data company.

Also, the start of tolling on the Highway 520 bridge on Dec. 29 created traffic delays of eight minutes at peak times on I-5, and slowdowns on I-405, as drivers detoured to reach toll-free I-90, state data showed in March. But the team from Amsterdam-based Tom Tom wasn't familiar with 520 tolling specifically.

Often the differences between cities are minor, yet the rankings fuel conversations and political rants.

Earlier this year, rival data firm INRIX ranked Seattle No. 7 in delays, using figures for all of 2011. The Texas Transportation Institute currently ranks Seattle 12th, based on 2010 data.

Whether we're fourth or only seventh, what the numbers say is that frustrated travelers here aren't delusional. Seattle endures volatile traffic and more congestion than most of its peers.

INRIX, based in Kirkland, says delay has actually lessened a bit since last year — "still slightly down, like our economy," says spokesman Jim Bak, who also blames Washington state's high gasoline prices.

Tom Tom found:

• The average Seattle-area driver wastes 35 minutes in slowdowns for every hour on the road during commute periods.

• Commute times are 48 percent longer in the busiest morning hour than in free-flowing traffic, a full 70 percent longer in afternoon, and an average 25 percent longer in a 24-hour day.

• Wednesday mornings and Friday afternoons are the worst times to travel.

The survey doesn't measure transit trips. Seattle is about the seventh-busiest U.S. transit market, and most commuters to downtown or the University of Washington don't drive alone but use a bus, carpool, train, bicycle, their feet or a ferry.

North America's 10 most congested cities, according to Tom Tom, are: Los Angeles; Vancouver, B.C.; Miami; Seattle; Tampa; San Francisco; Washington, D.C.; Houston; Toronto and Ottawa.

Honolulu and New York made the INRIX worst 10, but not Tom Tom's list. Cohn said its New York survey area includes suburban roadways that function well, while Honolulu simply wasn't among the 26 cities examined.

Tom Tom looks at a compact area — here, only as far out as Burien or Redmond — to avoid rural roads that dilute the results, said Cohn. INRIX measures all of King, Snohomish and Pierce counties.

Bak predicts that as tolling and construction spread in the Seattle area, the traffic here will continue to be unpredictable.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.