Before the present state highway numbering system was first posted in January 1964, an earlier numbering system had been used. The Washington State Department of Highways had been created in 1905. By the second decade of the 1900s, a numbering scheme had appeared on Department maps. This scheme used one and two digit numbers to identify state highways. The 1937 session of the state legislature created a secondary state highway (SSH) system. SSH numbers were numero-alphabetic, e.g., SSH 1A, 1B, and 1C branched off from PSH 1. The earlier state highways became known as primary state highways (PSH). I believe that the SSH numbers were first posted in 1938. US and Interstate highways had unposted state highway numbers that appeared on maps and in tables. The PSH and SSH numbers were replaced in the field on state highways in 1964 but remained as the official or legislative numbers until 1970, when the SR numbers replaced the PSH and SSH numbers in the Revised Code of Washington.
From 1923 to 1937, state highways had been known as State Roads. Most state roads had numbers as well as names, but some highways had names only, e.g., Methow Valley Highway. The State Road numbers became the PSH numbers in 1937. New numbers were assigned to those highways that had previously only identified by name.
Primary State Hwys Secondary State Hwys