Washington State Legislature (March 19, 1923). "Chapter 185: Primary and Secondary State Highways". Session Laws of the State of Washington (1923 ed.). Olympia, Washington: Washington State Legislature. pp. 627–628. Retrieved February 26, 2013.
Washington State Legislature (March 17, 1937). "Chapter 190: Establishment of Primary State Highways". Session Laws of the State of Washington (1937 ed.). Olympia, Washington: Washington State Legislature. p. 933. Retrieved February 26, 2013.
Washington State Legislature (March 18, 1937). "Chapter 207: Classification of Public Highways". Session Laws of the State of Washington (1937 ed.). Olympia, Washington: Washington State Legislature. pp. 996, 998. Retrieved February 26, 2013.
"SR 99: Junction SR 509"(PDF). Washington State Department of Transportation. March 31, 2011. Retrieved February 26, 2013.
Staff (2011). "2011 Annual Traffic Report"(PDF). Washington State Department of Transportation. pp. 190–191. Archived from the original(PDF) on June 13, 2013. Retrieved February 26, 2013.
Prahl, C. G. (December 1, 1965). "Identification of State Highways"(PDF). Washington State Highway Commission, Department of Highways. Archived from the original(PDF) on February 2, 2020. Retrieved February 26, 2013.
Staff (2011). "2011 Annual Traffic Report"(PDF). Washington State Department of Transportation. pp. 190–191. Archived from the original(PDF) on June 13, 2013. Retrieved February 26, 2013.
Prahl, C. G. (December 1, 1965). "Identification of State Highways"(PDF). Washington State Highway Commission, Department of Highways. Archived from the original(PDF) on February 2, 2020. Retrieved February 26, 2013.