 The Capital Construction Company began improving the county road between Colfax ( on Route 37 , signed as US 40 ) and Grass Valley ( on Route 17 ) under contract on September 29 , 1931 , and completed the work by mid @-@ September 1932 . The state legislature placed it on the state highway system in 1933 as an extension of the existing Nevada City @-@ <unk> Route 25 . In the 1964 renumbering , the highway received a signed designation , State Route 174 . 
