Let no one run away with the idea that the alternative is easy. I did manage to get from Kelso to Camas without going on I5 by attempting to follow old 99. It involved a very hard climb and a lot of time. Some gent who lives in Kelso sent me a PM saying I couldn't have chosen a worse route, I should have gone down I5.
Oh, believe me, I know. I went up that hill the first time I biked between Olympia and Portland in '08. I threw up. Since then, I've used I-5 between Woodland and Kalama. I normally take try to stay off the freeway if I can, but this is one place I make an exception.