Between the Mexican border and the Canadian border the narrowest area of I5 is right smack in the middle of Seattle, where it closes down from about 5 or 6 lanes northbound to only 2 lanes, before it picks up lanes again 1 at a time after the I90 interchange. And with the Convention Center built over the freeway at this choke point, there is no way to widen the freeway.
Between the Mexican border and the Canadian border the narrowest area of I5 is right smack in the middle of Seattle...
Ummmm..., you haven't seen I-5 in downtown Portland on the Eastside, then... LOL...
Oh..., and one more thing, the tightest curve on I-5, for its entire length (southern border to northern border) is right there in Portland... called the Terwilliger Curves... :-)
http://wikimapia.org/6258808/Terwilliger-Curves