In Texas a maximum speed limit of up to 85 mph (137 km/h) can be established on certain highways. 75 mph (121 km/h) is posted on federal or state highways, city maintained roads, and toll roads, and up to 70 mph (113 km/h) on county roads. In Texas the law generally prescribes a statutory speed limit of 70 mph (113 km/h) for any rural road that is numbered by the state or federal government — whether two-lane, four-lane, freeway, or otherwise — 60 mph (97 km/h) for roads outside an urban district that are not federal or state highways, and 30 mph (48 km/h) for streets in an urban district.
Texas is the first state to lower speed limits for air quality reasons, although the lowered limits may not meaningfully improve air quality. For example, the environmental speed limits in the Houston Metro area enforced with the exception of the tolled sections of the Grand Parkway with a 70 mph (113 km/h) speed limit – also a section of Interstate 10 between Winnie and Beaumont was lowered to 65 mph (105 km/h) in response to traffic fatalities.
Texas law allows 75 mph (121 km/h) speed limits on any numbered state highway or city maintained road and it’s the only state with 75 mph (121 km/h) limits on two-lane roads. Because speed studies undertaken by Texas Department of Transportation were made the result is that the mileage of highway with a speed limit of 75 mph (121 km/h) has increased from about 1,400 to about 19,000. 70 mph (113 km/h) speed limits have become rare on Texas interstates. They are retained in stretches of I-10 and I-35 in Bexar County, on I-410, and on I-35 from San Antonio to Austin.
Texas allows 80 mph (129 km/h) speed limits on I-10 and I-20 in certain counties named in the statute, each of which has a low population density. Additionally, the Texas Transportation Commission may set a speed limit up to 85 mph (137 km/h) on any part of the state highway system if that part is “designed to accommodate travel at that established speed or a higher speed” and an “engineering and traffic investigation” determines the speed is “reasonable and safe”.
In Texas the roads with an 80 mph (129 km/h) limit are: I-10 between mile 61.8 in Hudspeth County and mile 494 in Kerr County; I-20 between mile 0 in Reeves County and mile 89 in Ward County; State Highway 45 South from the northern junction with US 183 to the southern junction with I-35 in Travis County; SH 45 North from the northern terminus of its concurrency with SH 130 to the northern junction with US 183 near the Travis/Williamson County border remains at 75 mph; State Highway 130 from I-35 north of Georgetown to the northern terminus of its concurrency with US-183 south of Austin.
For now the only road in US with an 85 mph (137 km/h) speed limit is a 41-mile portion of Texas State Highway 130 from the northern terminus of its concurrency with US-183, southward to I-10 near Seguin and the speed limit goes through a mid sized urban area called Lockhart.