Washington State Legislature (March 19, 1923). "Chapter 185: Primary and Secondary Highways". Session Laws of the State of Washington (1923 ed.). Olympia, Washington: Washington State Legislature. p. 631. Retrieved June 12, 2009.
Washington State Legislature (March 18, 1937). "Chapter 190: Establishment of Primary State Highways". Session Laws of the State of Washington (1937 ed.). Olympia, Washington: Washington State Legislature. p. 1001. Retrieved June 12, 2009.
Washington State Department of Transportation (2007). "2007 Annual Traffic Report"(PDF). Archived from the original(PDF) on March 26, 2009. Retrieved June 12, 2009.
C. G. Prahl (December 1, 1965). "Identification of State Highways"(PDF). Washington State Highway Commission, Department of Highways. p. 12. Retrieved June 12, 2009.
Washington State Department of Transportation (2007). "2007 Annual Traffic Report"(PDF). Archived from the original(PDF) on March 26, 2009. Retrieved June 12, 2009.