Before 1964, Route 173 was part of California Legislative Route 59; however, like a small number of other California State highways, it was not signed. From 1934 to the mid-1950s the part of Route 59 that is now Route 173 was designated by the California Division of Highways to be a segment of California sign route 2; however, signing did not occur. From the mid-1950s through July 1964, the Division of Highways changed the routing of future sign route 2 from the subject segment of Route 59 to Legislative Route 188, the segment of present
California 138 between the junction of Routes 138 and 173 and Mount Anderson Junction, the junction of Routes 18 and 138 south of
Crestline. The highway has faced repeated problems since the 2003 Willow Fire that has made the one-lane unpaved trail portion (approximately between Postmile 7.5 and Postmile 11.5) unsafe for passage from erosion and storm damage. Since March 2011, the one-lane trail portion of SR 173 has been permanently closed to through traffic. While roadway preservation maintenance (basic grading, debris clearance) is still done, this decision effectively ends all further interest to upgrade the segment to a 2-lane passable highway through state-funded projects. ==Major intersections==