High Quality Content by WIKIPEDIA articles! Interstate 605 (I-605) is the common designation given to several proposals for a new auxiliary route of the Interstate Highway System bypassing Interstate 5 (I-5) and I-405 in the US state of Washington. Proposals have been heard from since the 1960s, including highways connecting from I-5 all the way to the Canadian-American border, and some just between I-5 and I-90. The Washington State Department of Transportation (WSDOT) has no projects designated I-605 at this time. However a project to convert State Route 18 (SR 18) to a limited-access freeway is under construction currently.