SR 171 starts at I-90 Exit 175 on the SW side of Moses Lake and heads NE via Downtown Moses Lake to Jct SR 17. From I-90 to downtown, SR 171 is also posted as Business Loop 90. The description of SR 171 in the Revised Code of Washington states that SR 171 is to continue in a NE direction from the Moses Lake area to Jct SR 28 on the west side of the Odessa area. Except for a few miles of road not maintained by the state, the SR 17 to SR 28 portion of SR 171 is still proposed and might never be built. SR 171 from I-90 to SR 17 is 3.82 miles long.
Before 1964, SR 171 was numbered SSH 7E.
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