When I lived in Seattle, it was amazing how slow people drive. And a little rain/snow and nobody knows how to drive.
Having lived all over the country I've come to the conclusion there are two big reasons: lifestyle and outlook.
People in the northwest are more laid back than most other areas of the country. The south claims a leisurely lifestyle, but it's honestly more a combination of heat exhaustion and lethargy than leisure. Up here it's not laziness or slowness, just a calm peace that so many seem to share. The pacific northwest is like a big Taoist commune or something. That seems to translate into less need for speed in the car.
The other part, outlook, is a little tougher to see...especially in the last ten or so years as things seem to be moving more towards the rest of the country. Basically people just seem less self-centered, or more aware that they're sharing the situation with others. My time in California, the deep south, the east coast...all were punctuated with people who seemed to not realize that there was anyone in the world but themselves. They drove aggressively, talked aggressively, focused all their energy on imposing change on their environment (change their income, their career, their social status, etc). Up here, we seem much more likely to just be who we are and enjoy what already is. When I talk to my cali buddies about driving they're all 'you're slowing ME down'. When I talk to hardcore pacnw'ers they're more like 'hey however you want to be, you have as much right to the road as I do'.
Obviously it's not an absolute...I met laid back people all over, and there are A types here...but in general, it's what I've noticed (and why I will only live up here now).