In Vancouver, I stood in the apex of I-5 and an off-ramp once, got me a ride all the way to Seattle. Illegal, but I recall being passed by a small handful of law enforcement vehicles, none of which bothered me.
Also have used an I-5 on-ramp in Vancouver, the 39th Street one, that got me to Federal Way, WA. The on-ramp at E. Mill Plain Blvd. looks like it might pay-off.
Or how about the on-ramp from Jantzen Beach/Hayden Island, OR? Talked to a guy I saw waiting there, and eventually he got a ride somewhat north...