Last September two of us rode from Aberdeen, WA to San Francisco. It was the second time for me & the third time for him. Next year our plan is to start a TransAmerica ride, but break it up into three years. We are both quite married, and breaking up the ride is the only way we can do it. We are investigating routes now. I would like to leave from Astoria, OR as that is only a 2 day ride from where we live, or we could be driven there to start. Adventure cycling has the Lewis & Clark Route which intersects the Northern Route on the east edge of Montana. What do you think of that as a plan? We would be giving up our cushy motel only travel style for this journey.
That's the general route I took on my first cross-country ride way back in 1987.
The roads are a lot busier now - but comparable to some you road on along the coast.
(I've biked parts since then - esp. around Cathlamet - but the corridor is too developed for me.)
Personally I don't think the Portland option on the AC maps is the best.
I'd suggest crossing the Columbia on the Cathlamet Ferry and riding on the north bank of the Columbia.
From Longview there is a back road to Kalama -
Then is a 3-mile section where you have to ride on the I-5 shoulder -
Then back roads thru La Center, Hwys 502, 503, 500 then Washougal Valley Rd to Hwy 14.
This route lets you avoid Portland and Vancouver plus enjoy the ferry ride on the river.
If you ride on the Oregon side there are numerous sections where you have to ride on I-84.
Hwy 14 on the Wash side is much better - plus there are state parks at convenient intervals.
I don't know why - but Wash Parks don't have hiker/biker sites like Ore & Calif.
US 12 over Lolo Pass is part of the TransAm route and very lovely.
Don't overlook Jerry Johnson Hot Springs - make it an overnight stop.
Then Hwy 200 in Montana is O.K. - a bit busy for me - make sure to stop in Ovando.
Not sure why they send you up to Augusta rather than down to Wolf Creek and along the Missouri River.
Recreation Road along the River is a really fine ride.