Although presented in digitized format here, all of these publications are available in print format from the WSDOT Library collection.
Development of Washington State Highways and Transportation (pdf 840 kb)
A History of Highways: Washington State Highway & Transportation Commission 1951-1993 - Washington State Highway & Transportation Department 1905-1993
40 Years With the Washington State Department of Highways (pdf 100 kb)
An outline of the growth and development of the Washington Department of Highways from its creation in 1905 to the close of World War II in 1945.
History of Roads & Highways in the State of Washington (pdf 2.6 mb)
An early history of roads in Washington state, from territorial days to the 1930s.
Identification of State Highways (December 1, 1965)
Includes two tables; one converting Legislative highway numbers to corresponding SR numbers, and the other converting SR numbers to corresponding Legislative highway numbers.
Available from the Washington State Library
Historical Washington maps, including highway maps from 1909, 1912, 1915, 1931, 1933, 1939, and 1950.