SR 906 is similar to SR 904 in that the number was assigned to a portion of US 10 but not posted until a few years later when the highway was bypassed by I-90. The SR 906 designation was assigned to US 10 from the I-90 west summit interchange (Exit 52) near Snoqualmie Pass to the Hyak interchange (Exit 54) in 1967. The I-90 freeway over Snoqualmie Pass opened in late 1969. A spur of SR 908 links the SR 908 mainline near Hyak with the WSDOT Hyak maintanence shed. SR 906 is 2.65 miles long. SR 906 Spur is 0.43 miles long.
The unposted legislative number assigned to SR 906 was SSH 2K.
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