Longview probably just has the one zip code. Seattle has 59, for just over 10000 average per zip code, so we up here are probably a bit worse in a big city.
Worse or better? With only 10k per zip you'd get very close to a 100% unique identifier.
I realized after looking at both Portland and Seattle zip codes that they seem to be distributed better than the less dense areas so you do have very few zip codes over 20k.
