That is until you hit the bridge into Oregon. Then there's tons of traffic, negative scenery, and lots of roadside flavor you might want to do without.
Well I wouldn't go as far as to say negative
scenery, but I do agree about the tons o' traffic on the STP route once you cross the Lewis and Clark (Longview), as the route follows busy US 30.
There is an alternate route into Portland that connects to the STP route at Kelso/Longview, which is on the Washington side of the Lewis and Clark bridge:http://www.bikely.com/maps/bike-path/Vancouver-to-Longview-WA
This route stays on the Washington side of the Columbia and crosses into Portland at Vancouver (Interstate Bridge).
I've ridden this route once, and it is pretty nice, except for one big exception: the hill just south of Kalama. It is an intense, "I hate my life" type of hill. If you wanted to experience "Appalachian Grade" hills on the West Coast, here's your chance. The only way to detour it is by hopping on the shoulder of I-5 (yep, the freeway) for about 5 miles.
And here's a routing for the STP:http://www.bikely.com/maps/bike-path/Seattle-to-Portland
I'm planning on biking up to Seattle soon and use most of the STP route, but want to detour at Tacoma and take the ferry over to Vashon Island. Has anyone done that route before?