Portland is a national success story when it comes to reducing traffic congestion, according to an analysis released this month by the navigation company TomTom.

According to the company’s updated traffic Index, Portland had the nation’s 15th worst traffic last year, but Rose City drivers saw a 3 percent reduction in total congestion from 2017 to 2018, the best improvement in the country.

Portland had the 11th worst overall traffic congestion in 2017. For a 30-minute commute, drivers spent an extra 17 minutes in their cars in evening rush hour last year, a 1-minute improvement from the previous year. Morning rush hour travel times remained stagnant.

The modest improvement seems to buck the conventional wisdom espoused by local, state and regional government that the Portland region’s traffic woes are only worsening as the population continues to grow throughout the broader Portland-Vancouver area.

Not so fast, TomTom officials said. While Portland’s improvements were slight, Salt Lake City is the only other U.S. city to see comparable reduction in traffic congestion.

“Given the large data sample size and direct way of measuring travel time differences based on accurate GPS measurements, we stand behind the results,” said Nicholas Cohn, TomTom’s senior traffic expert.

The navigation company cited Portland and Salt Lake City’s ongoing efforts to reduce traffic in a number of ways as a key factor, including spending “heavily in sophisticated traffic light optimization, bike infrastructure, light rail transit, and reductions to parking availability.”

TomTom collects the data from its vehicle navigation systems and smartphones. The company declined to share how many devices are used to provide the traffic analysis, but it said it crunched 1,224,061,938 driven miles in the Portland area to calculate its rankings.

The company said it defines congestion by measuring the fastest travel times, known as free-flowing conditions, then comparing all travel times points across roads throughout the day to determined how much extra time, on average, motorists spend in traffic.

A traffic jam exists when vehicle speeds drop below a specific threshold compared to free-flowing speeds. For highways the company uses roughly 60 percent and non-highways 50 percent.

“We are aware that there have been many road, public transit and cycle improvement projects in Portland (and potentially much larger ones being planned),” Cohn said in an email. The Portland region expects to see a massive transportation bond in 2020, the centerpiece of which is expected to be a 12-mile light rail extensive through Southwest Portland to Bridgeport Village.

Cohn said big construction projects and a rainy and snowy 2017 in Portland may have influenced travel times as well.

Joe Cortright, an economist who runs a blog where he discusses transportation issues, said the TomTom users may be super commuters who drive a lot, so the data may not be representative of all car commuters. He’s also skeptical of congestion-related rankings, saying they often ignore the variance in commuting distances and can lead to skewed statistics, where compact cities where commuters have shorter distances appear to have worse congestion than travelers who must commuter from far-flung suburbs.

But Cortright said on his blog the traffic congestion rankings would seem to merit more attention from state transportation agency’s to projects like congestion pricing, which is a proven way to reduce traffic.

“But building things, not solving traffic problems, is really their priority,” he said of state transportation departments.

Oregon is planning to charge drivers to use sections of Interstate 5 and 205 in the Portland region, but that is likely years away.

ODOT officials declined to comment on the specific TomTom report, but said the agency is working throughout the metro area to add auxiliary merging lanes to help traffic move more efficiently on the interstates.

Dylan Rivera, a Portland transportation spokesman, said no matter where Portland falls on the rankings, it is trying to address major transit issues across the city. “We’re working hard to provide a transportation system that helps people get where they need to go safely, reliably and sustainably,” Rivera said in an email. “We will continue to reduce carbon emissions and grow our economy by investing in the public transit, biking and walking access that also improve our quality of life and help Portland households spend less on transportation.”

-- Andrew Theen

atheen@oregonian.com

503-294-4026

@andrewtheen

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