Vancouver-area traffic congestion has taken a steep drop since 2009, despite an overall increase in Washington, according to a state report released this week.
Explaining those disparate trends, however, is more hairy than the Interstate 5 Bridge at rush hour.
"There isn't one factor," said Abbi Russell, a Washington State Department of Transportation spokeswoman. "There are lots and lots of reasons why this could have happened."
Some analysts see traffic congestion as something of an economic indicator. The theory goes like this: The more the economy picks up -- prompting more people to get in their cars and drive to work, go shopping or run errands -- the more clogged major roadways become.
Of course, more traffic also comes with an economic cost through wasted fuel and lost business productivity, according to the study.
In Washington, overall congestion numbers are driven almost entirely by the Puget Sound region, which accounts for 99 percent of traffic delay hours in the state, according to the study. Vancouver, the Tri-Cities and Spokane account for the other 1 percent.
The research showed that Washington drivers overall spent about 16 percent more time staring at brake lights in 2011 than they did in 2009. The increase was less pronounced in the second year. But in Vancouver, traffic went sharply the other way -- delay hours dropped by more than half during the same time period. Motorists on local roads saw the number of delay hours fall from 1,090 hours to 459 hours in those two years.
It's tough to pin that decline on one single reason, officials said. Economic activity can play a role. So can emergency incidents, crashes, weather and construction. As a border city, Vancouver's major highways are also connected to a larger network in another state, said Sreenath Gangula, a WSDOT systems analyst who worked on the congestion study.
"There are a lot of factors that go into (congestion), and I can't hang my hat on one," Gangula said, noting WSDOT simply doesn't have the amount of data for Southwest Washington that it does for the Puget Sound network.
At least one recent project in Portland may have helped reduce Vancouver's congestion, Russell said. The Oregon Department of Transportation in 2010 finished widening a section of Interstate 5 at Delta Park -- a stretch thousands of Clark County residents drive through each day on their way to work in Portland. The result may have alleviated southbound congestion that routinely stretches across the Columbia River into Vancouver, Russell said.
A number of WSDOT projects are under way but unfinished in Clark County, including work on state Highway 500 in Vancouver, state Highway 14 in Camas and Washougal, and the northern convergence of I-5 and Interstate 205 in the Salmon Creek area.
None of those would have affected Vancouver's drop in traffic delays from 2009 to 2011. Crews did carry out other congestion relief efforts, but nothing that would explain a drop as big as the study showed, Russell said.
Vancouver traffic bucked the statewide trend even before the most recent swing. From 2008 to 2009 -- while the economy cratered and the Great Recession searched for its bottom -- Washington's overall traffic congestion saw a big drop. But Vancouver again didn't follow suit, seeing an increase in traffic delays instead, according to the study.
Regardless of its cause, having vehicles stacked up on highways is a negative for the economy, said Scott Bailey, a regional economist with the Washington Employment Security Department. Congestion is a more direct indication of how transportation infrastructure is performing than how the economy is performing, he said.
Clark County's unemployment rate has hovered well above the state average in recent years, according to state data. It's also been well above double-digits for most of that time. The congestion study showed delay hours increasing in Vancouver from 2007 to 2009, then dropping from 2009 to 2011. Vancouver's 2011 congestion was lower than any of the previous four years.