Have a look at this page in Crazyguyonabike. http://www.crazyguyonabike.com/doc/2616
Washington most certainly does not allow cyclists on the interstates except where an alternative route does not exist
The whole "except where an alternative route does not exist" isn't really true anymore, especially in Washington State. This idea may have lifted prohibitions on bicycles on Interstates way back when. But when you look at the actual prohibitions on cycling on Interstates and other highways in Washington, you see that you can ride on I-5 or I-90 for great distances. With a few exceptions, the prohibitions are in urban areas where there are many on/off ramps and sometimes stretches of highway with no shoulder.
Take a look:
For example, you can ride the shoulder of I-5 for the approx. 100 miles between Olympia and Salmon Creek (outside of Vancouver, Wash.) without having to get off the Interstate. And I know for sure there are "alternative routes" that parallel I-5 here (Old 99, for one), yet you can still legally ride the shoulder of I-5 if you want.