Farm to Market Road 170

The earliest portion of the present route to be designated as part of the state highway system was the section between Terlingua and Study Butte which was assigned in September 1939 as part of a spur to SH 227. This spur continued on to connect with the main highway at the present junction of the north end Old Maverick Road and the road to Panther Junction within Big Bend National Park. SH 227 at the same time was newly designated as a road from Santa Elena Canyon to Panther Junction via the Old Maverick Road and then to Persimmon Gap and Marathon along the present US 385.[2]

One month later, this spur was reassigned as part of SH 118.[3] At that time, SH 118 south of Alpine departed from its present route near Adobe Walls Mountain to Terlingua rather than going directly to Study Butte as it does today.[4] From Terlingua, SH 118 then proceeded eastward through Study Butte to SH 227.[3]

In 1941, the designation of SH 118 along the road between Alpine and Terlingua was canceled and returned to county maintenance. The road from Terlingua through Study Butte to SH 227 was re-designated as Spur 121 until that designation was canceled in 1946. The portion within the newly created Big Bend National Park was incorporated into the park's road system. The remainder between Terlingua to the park boundary near Study Butte reverted to county maintenance.[5]

By 1951, SH 118 had been incrementally extended south of Alpine to the park boundary along its present alignment through Study Butte and over the portion of the former spur road to the park boundary. State maintenance between Terlingua and Study Butte was reestablished as RM 2462 in 1956. RM 2462 was then extended westward to the Presidio County line just west of Lajitas in 1957.[6]

FM 170 was originally designated as a 5.0-mile (8.0 km) highway from US 67 in Presidio northwest to Ochoa. On December 16, 1948, the road was extended to the northwest an additional 13.9 miles (22.4 km) to Adobes and to the southeast to a point 6.3 miles (10.1 km) from Presidio. On July 15, 1949, the road was extended 11.7 miles (18.8 km) to Redford. The road was extended on October 29, 1953 further to the northwest 13.9 miles (22.4 km) to Pinto Canyon. Further construction to the northwest would then undergo a long hiatus.[7]

On July 28, 1955, with construction focusing toward the southeast, the road was extended 5.2 miles (8.4 km) beyond Redford. On October 31, 1958, the road was extended an additional 13.9 miles (22.4 km), and FM 170 was changed to RM 170.[8] On February 5, 1960, RM 170 received its final extension to the southeast to RM 2462 at the Brewster County line. The RM 2462 designation was then canceled with that road combined with RM 170 for a total extension of 38.0 miles (61.2 km) to its present terminus at SH 118.[7]

With southeastward road-building complete, expansion to the northwest resumed on June 2, 1967 with an 8.5-mile (13.7 km) to Ruidosa. Between 1967 and 1981, RM 170 was changed to FM 170. Ruidosa would remain the northwest terminus for fourteen years until the road was finally extended an additional 11.6 miles (18.7 km) to its present terminus at Candelaria on October 21, 1981. Also, sometime between June 2, 1967 and October 21, 1981, RM 170 was changed to FM 170.[9][7][10]

On June 18, 1996, FM 170 was rerouted on Presidio's north side to follow concurrently along the route of US 67 and Bus. US 67-A abandoning a short section of road east of US 67. This was due to the relocation of US 67 away from the town center toward the Presidio-Ojinaga International Bridge with much of US 67's former route through town being replaced by the newly designated Bus. US 67-A, Spur 203, and Spur 310.[7][11][12][13]