Continuing my obsession with the ever-mounting evidence that traffic volumes are growing much, much slower than transportation planners had expected, I present to you: Traffic volumes on the I-5 and I-205 bridges across the Columbia River, with data courtesy of the Washington State Department of Transportation’s Annual Traffic Report series.

In case it’s not obvious from the chart, traffic on the two highways combined was a wee bit lower in 2010 than it was in 2003. Much of that decline was due to the sharp drop-off in 2008, when gas prices were high and the economy tanked.

But if you look even closer at the numbers, you’ll see something else: in about 2000, the growth in traffic over the Columbia started to stall out. Before then, traffic grew by an average of about 5 percent per year. Since the new millennium, though, annual traffic growth has averaged about 0.5 percent. Even if you exclude the declines in 2008, traffic grew at only about 1 percent per year starting in 2000.

That’s for the two highway bridges together. If you look just at the I-5 span across the Columbia, traffic volumes in 2010 were just a hair above what they were in 1999. That’s more than a decade with essentially no growth in traffic!!

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Of course, even slow growth can add up, if you wait long enough. Still, the oft-cited rational for the Columbia River Crossing megaproject—an expansion of the I-5 span that’s projected to cost between $3.2 billion and $3.6 billion—is that traffic is growing so rapidly and so inexorably, that we have to spend exorbitantly to prevent congestion from miring the Portland metro area in perpetual gridlock.

But the projections of rapid traffic growth come primarily from transportation models. And those models essentially have rapid traffic growth built in as core assumptions—which means that rapid traffic growth isn’t so much a conclusion of these models as a premise.

Up through the 1990s, that premise was more or less true: year after year, you could count on steady increases in traffic volumes across the Columbia. But for the last decade or so, the world just hasn’t worked that way. And as a result, the idea that we have to build more and more lanes to accommodate the crush of new traffic is looking shakier with every passing year.