High Quality Content by WIKIPEDIA articles! U.S. Highway 181, a north–south route, is one of many intrastate United States Highways in the United States. It lies completely within the state of Texas, and both termini are at Interstate 37, the road that it mainly parallels to the east. US 181 begins in Corpus Christi, Texas at mile marker 0 to the south, through south-central Texas to just south of San Antonio to the north (mile marker 131), for a total length of 137 miles (220 km).