State highways in 1970: primary in red and secondary in purple
Primary State Highways were major state highways in Washington state. They were used from 1905 to 1964. The 1964 state highway renumbering changed the highway numbering in the state to consolidate and create a more organized and systematic method of numbering the highways within the state.
The first state road, running across the Cascade Range roughly where State Route 20 now crosses it, was designated by the legislature in 1893 (However, this road wasn't actually opened until 1972). Two other roads - a Cascade crossing at present State Route 410 and a branch of the first road to Wenatchee - were added in 1897. The Washington Highway Department was established in 1905, and a set of twelve State Roads, numbered from 1 to 12, were assigned. A thirteenth was added in 1907, and State Roads 14 to 18 in 1909.
However, it was not until 1913 that a connected system was laid out - earlier state roads had been disconnected segments of road needing improvements. The seven primary roads were only assigned names, while the older state roads kept their numbers as secondary roads. In 1923, most state roads were assigned new numbers, though the primary and secondary split remained, and several roads remained named only. The United States Numbered Highways were assigned in late 1926, overlapping some of the State Roads.
The primary/secondary state highway systems was replaced by the current numbering system in the 1960s. The signs for the new highway numbers first were posted in 1964, while the PSH/SSH signs were removed in 1970.
This route followed Interstate 82 from Ellensburg to the Oregon State line (concurrent with U.S. Route 97 from Ellensburg to Union Gap and U.S. Route 410/12 from Union Gap to the Tri-Cities), U.S. Route 410 (now U.S. Route 12) from the Tri-Cities to Clarkston, U.S. Route 195 from Clarkston to Pullman, SR 27 from Pullman to Oaksedale, US 195 from Oakesdale to Spokane, U.S. Route 2 from Spokane to Mead and U.S. Route 395 from Mead to the U.S.-Canada border. Spurs extended along I-82/, SR 125 and SR 129 from Tri-Cities, Walla Walla and Clarkston to the Oregon State Line
This route was also known as the "Inland Empire Highway" and crossed the first, and for a time, the only highway bridge over the middle Columbia River. That bridge was located between the towns of Kennewick, WA and Pasco, WA.