By-Passing Portland:
Portland has two possible belt loops. One, I-205, goes between exit 288 on the Oregon side, up the eastern side, and reconnects with I-5 at exit 9 on the Washington side.
Another one has a few more roads involved, but means you will almost-completely by-pass Portland. If you're heading north to Seattle, or south to Eugene, perhaps you don't care about Portland. Then you use the following routes:
Southbound: Get off I-5 at exit 39 in WA, and go west on WA-4 into Longview. Follow the signs to the Lewis & Clark Bridge to Rainier/US-30. Cross the river on said bridge, then go east on US-30. Follow this route past Scappoose, about 5 miles, and turn south (right) on NW Cornelius Pass Road. You are on that route for about 8 miles - a 2-lane road through a beautiful forest and farm lands, over a "pass" of about 500 ft elevation, no danger. You will come to US-26, which you take east for about 7 or 8 miles to OR-217. Take that south to I-5 South.
Northbound: I-5 north to the OR-217 north at exit 292. Then take US-26 west to the exit for NW Cornelius Pass Road. After 8 miles, turn west on US-30 through Scappoose and Rainier, then use the Lewis & Clark Bridge over the Columbia into Longview. Follow the signs to I-5 or signs to Kelso, where you pick up I-5 north.
It adds NO mileage, does take about 10 minutes longer because of US-30 (2 lane), but if you are traveling mid-day on a weekday, or near a rush hour, it saves time (except to get on the Lewis & Clark Bridge, which gets busy at rush hour).
Donna