SR 100 was added to the state highway system effective April 1, 1992. This route begins in Ilwaco at Jct US 101, heads west, then south to Fort Canby State Park, then east, and north to rejoin itself a few blocks west of US 101. The word LOOP is signed instead of cardinal directions on SR 100. SR 100 spur starts beyond the state park and heads south to a Coast Guard facility. SR 100 is 4.68 miles long. SR 100 Spur is 0.61 miles long.
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