Every year, the Washington State Department of Transportation (WSDOT) conducts a series of surveys on its highways in the state to measure traffic volume. This is expressed in terms of annual average daily traffic (AADT), which is a measure of traffic volume for any average day of the year. In 2011, WSDOT calculated that the busiest section of the highway was between SR 3 and Liberty Bay, serving 11,000 vehicles, while the least busy section was the bridge over Liberty Bay, serving 6,300 vehicles.[5]
^Staff (2011). "2011 Annual Traffic Report"(PDF). Washington State Department of Transportation. p. 173. Archived from the original(PDF) on June 13, 2013. Retrieved February 17, 2013.
^Washington State Legislature (March 14, 1929). "Chapter 116: Primary State Highway No. 21 to Keyport". Session Laws of the State of Washington (1929 ed.). Olympia, Washington: Washington State Legislature. p. 248. Retrieved February 17, 2013.
^Washington State Legislature (March 17, 1937). "Chapter 185: Establishment of Primary State Highways". Session Laws of the State of Washington (1937 ed.). Olympia, Washington: Washington State Legislature. pp. 941–942. Retrieved February 17, 2013.
^Seattle, 1958(JPG) (Map). 1:250,000. United States Geological Survey. 1958. Retrieved February 17, 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 October 30, 2008. Retrieved February 17, 2013.
^Seattle, 1965(JPG) (Map). 1:250,000. United States Geological Survey. 1965. Retrieved February 17, 2013.
^Staff (1970). "Annual Traffic Report 1970"(PDF). Washington State Highway Commission, Department of Highways. p. 169. Archived from the original(PDF) on October 17, 2013. Retrieved February 17, 2013.
^Staff (1991). "1991 Annual Traffic Report"(PDF). Washington State Department of Transportation. p. 129. Archived from the original(PDF) on January 2, 2014. Retrieved February 17, 2013.