The number of Clark County residents who worked in Oregon peaked in 2008, at the start of the Great Recession. From 2002 to 2011 (slightly beyond the scale of this graph), the county population grew by 17 percent.
C-Tran ridership on Portland commuter routes, 2002 to 2011. The cost of a monthly commuter pass doubled in 2005; the 99th Street Transit Center opened in 2008.
In a decade of growth in Clark County, a bedroom community where a third of the workforce commutes to Portland, you'd expect to see traffic on the bridges crossing the Columbia River to worsen.
Traffic rose just 1 percent from 2002 to 2011, according to the most recent counts available, even as the county's population grew 17 percent.
Critics of the Columbia River Crossing project have argued these figures cast doubt on traffic forecasts used to justify plans for a $3.5 billion new Interstate 5 bridge.
The project's environmental impact statement estimates that, if no action is taken, 184,000 cars a day will cross the I-5 bridge in 2030. Planners say daily traffic congestion -- which engineers define as freeway traffic that moves at less than 30 mph -- will increase, from four to six hours a day on I-5 to 15 hours in both directions by then.
Yet over the past few years, drivers have taken their feet off gas pedals for a variety of reasons -- high fuel prices, a poor economy and generational shifts.
"People are driving less," said Portland economist Joe Cortright, a critic of the bridge project.
Planners estimated 143,700 vehicles would cross the I-5 bridge on the average weekday in 2010 --13 percent higher than the 126,900 actually experienced, Cortright points out.
Project planners look past blips in the near term to focus on "long-range needs in the corridor," Mandy Putney, spokeswoman for the project, wrote in response to The Columbian's inquiries. "Traffic forecasts are not based simply on projecting recent trends, but on developing future forecasts of transportation use based on other variables including population and employment forecast."
Long-term aside, what has happened to those drivers in the short term?
The Great Recession certainly played a role. Workers lost their jobs in years of record layoffs.
"Even those employees who remain employed tend to reduce their discretionary spending. This reduces travel for the purposes of shopping, entertainment and various other activities," Putney said. "Part of what we have seen in the traffic counts in the I-5 corridor is a reduction in the amount of off-peak travel, which is consistent with reductions in discretionary trips. Traffic volumes during the peak period, which includes most of the commuter work trips, have remained nearly constant."
Scott Bailey, a Washington state regional labor economist, agreed that the economy has dragged down driving.
"With the recession, there was a big drop in miles driven, with very little recovery there," Bailey said. "Clearly, commuters were hit by job losses. Bridge traffic is not just commuters. It's also shoppers, people visiting family, business trips and freight."
The economy isn't the whole story, however. Traffic started to level off before the economy officially crashed in December 2007.
"The recession argument doesn't wash. The decline in traffic on the I-5 bridge began well in advance of the recession, and traffic continued to decline even after the economy got better," Cortright said. "The long-term trend has everything to do with the fact that gasoline cost in real dollars has doubled."
The real price of gas declined from 1980 until 2000, and then started climbing, Cortright said.
"People respond to prices in a whole series of decisions: how much they drive, what kind of vehicles they drive, how far they travel, how far they look for jobs and houses," Cortright said.
Since the economic downturn, those who remained employed weren't likely to tinker with their jobs, Bailey said.
"I think with gas prices, most of the adjustment is going to be going toward a better-fuel-economy car. Jobs are a lot harder to come by. If anything, people would be willing to increase their commute to get a job," Bailey said. "There was actually an increase in the number of Portland people commuting to Vancouver. That shows gas prices aren't the big deal. It's, 'Where can I find a job?'"
Other factors also influence traffic flows on the I-5 bridge.
"Young people are driving less, and fewer are getting driver's licenses -- or waiting longer to get them. When you look at young adults, they are increasingly moving to city centers," Cortright said.
And, on a more prosaic note, don't forget orange cones.
The I-5 widening project at Delta Park in Oregon disrupted traffic flow from spring 2008 until its completion in 2010, Putney said.
Transit doesn't seem to be a big factor in bridge traffic counts, however. Droves of drivers haven't jumped out of their cars and onto buses.
"We have seen people giving it a try and sticking with it, but the numbers aren't significant," C-Tran spokesman Scott Patterson said. The transit agency's express commuter service has seen big changes over the last decade. Ridership dropped in 2005 when the cost of monthly passes for commuter routes to Portland doubled, then climbed temporarily in 2008 when C-Tran opened the 99th Street Transit Center.
"Really to make a significant impact, we'd need to provide a much higher level of service, and we don't have funding to do that," Patterson said.
Boosting transit ridership across the river would take something more drastic -- like light rail, one of the most controversial aspects of the I-5 bridge project.