When the current state highway numbering system was first posted in January 1964, the route that is now SR 166 was designated as part of SR 160. Effective April 1, 1992, SR 160 was moved to Sedgewick Road from SR 16 to Southworth. Former SR 160 from Jct SR 16 to Port Orchard was added back to the state highway system in 1993 as SR 166. The fact that the highway west of Downtown Port Orchard is prone to slides was probably the main reason that SR 166 was returned to state jurisdiction. The eastern terminus of SR 166 is described in the Revised Code Of Washington as the eastern city limits of Port Orchard. In theory, the city could annex east to Southworth and extend SR 166 along the entire length of former SR 160. SR 166 is 4.93 miles long.
The first roundabout on the state highway system is at the junction of SR 166 and Bethel Road east of Downtown Port Orchard.
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