When the current state highway numbering system was first posted in January 1964, SR 903 began at Jct US 10 in Downtown Cle Elum and headed northwesterly via Roslyn to the national forest boundary near Ronald. In 1967, I-90 from Exit 84 to Exit 106 opened to traffic, bypassing Cle Elum to the south. SR 903 was extended east from Downtown Cle Elum along former US 10 to Jct US 97 near I-90 Exit 85. This section of US 97 was renumbered SR 970 in 1975. Today, SR 903 is 10.06 miles long.
Before 1964, SR 903 had been SSH 2E.
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