City traffic operations manager Eric Widstrand confirmed this morning that traffic volumes in Seattle generally went down in 2008, as we reported earlier this week, but says the city's transportation planners have no immediate plans to shift their planning methodology, which assumes an annual growth in traffic of about one percent.

That assumption is important because it frames the way the city plans for growth—more investment in car infrastructure if there's more demand for roads, or more investment in the bike and pedestrian plans if people are shifting from cars to bikes and walking. For example, as I noted Tuesday, it makes little sense to build a new waterfront freeway and an expanded 520 bridge if all that new road capacity isn't necessary.

"Historically, you expect traffic to increase year after year" as population grows, Widstrand says. "Our engineers tend to be pretty conservative, so if a developer’s coming in and wants to do a traffic study, we still are going to require that" one-percent assumption.

In recent years, the city has expanded biking and pedestrian options, transit has improved, and gas prices have risen, a trend that doesn't seem likely to reverse itself. Given that people are driving less, doesn't it make sense to shift planning toward bike, pedestrian, and transit infrastructure (and away from cars)?

Widstrand says the trend will have to hold for many years before the city considers reworking its assumptions about traffic growth. "I would want to see if we're seeing declines of a decade or more before making a policy change like that," he says. "Are people changing their mode of travel? Are we seeing more people walking and biking? Mode shift overall is going to be a key indicator of that."

However, Widstrand says the fact that traffic didn't screech to a halt downtown during two major recent repaving projects (on Second and Fourth Avenues) is a good sign. "It's not causing a complete breakdown in the network, which shows that there's some excess capacity" in the system, he says.