How bout that two other Western WA people on here. I agree with the point that building more roads just equals more traffic equals more roads and so on. That's why I'm keen on the idea of time tiered tolls on I5, 520, 405, and I90 near Seattle. Charge enough to reduce traffic to make it flow easier. Perhaps no tolls for HOV people.
Another aspect of the traffic in Seattle is the total lack of infrastructure to cope with blips like accidents. I5 pretty much shuts down if you have a wreck. And various agencies (Qwest, installing traffic lights, widening the sidewalk, building construction) think *nothing* about shutting down a lane. Hey it doesn't cost them a thing. Meanwhile another 10 minutes has been added to my commute.