I just stumbled upon this chart that illustrates the relationship between density and driving.
Left axis is household vehicle mileage; bottom axis is metroplitan density. If you squint, you can find Seattle right in the middle of the cluster. Portland is about the same density, but with higher household mileage. You can find a bigger and clearer version on page 6 of this pdf.
Admittedly, this chart may be old news to some folks: the study was published in 2000, and it uses 1995 data. And though you can see the correlation pretty clearly, density obviously isn’t the only force in play. Nonetheless, the study found:
Suburban households drive 31 percent more than their urban counterparts, and western households drive 35 percent more miles than northeastern households.
Anyway, I thought it was a pretty snappy depiction of how land use may be in the driver’s seat for, uh, driving.