This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
Bicycles are permitted on Interstate highways in Washington. WSDOT permits bicycle use on the shoulders of most Interstate segments in Washington (including substantial portions of I-5, I-82, I-90 and I-182) where no reasonable alternative parallel route exists. Specific urban segments through Seattle, Tacoma and Spokane, plus several bridges and tunnels, are signed prohibited; verify against WSDOT's Bicycling on State Highways and Interstates guidance.
What this means in practice
Interstate cycling is legal in Washington, but ride only where you have a wide, clean shoulder and exit at the first signed bicycle exit when conditions deteriorate.