 State Route 174 begins at exit 135 of I @-@ 80 in Colfax . Immediately after crossing I @-@ 80 , the route turns right on Auburn Street and right again on Central Street , which bypasses downtown ; a business route follows Auburn Street , Grass Valley Street , and Main Street through downtown . SR 174 follows the old Lincoln Highway ( US 40 ) north out of downtown to Rollins Lake Road , where the present state highway turns northwest along the Colfax Highway . It then crosses the Bear River from Placer County into Nevada County on a 1987 bridge , with the old 1924 bridge preserved for non @-@ motorized traffic . The land flattens out as SR 174 curves north and west through the Empire Mine State Historic Park into Grass Valley . Turning west on Colfax Avenue , the state highway passes under the SR 20 / SR 49 freeway before ending just beyond at Auburn Street , the former surface routing of SR 49 . 
