State Highway 16 (SH 16) is a north–south state highway that runs from Zapata on the Texas-Mexico boundary to U.S. Highway 281 24 miles (39 km) south of Wichita Falls. It is the longest state highway in Texas at almost 542 miles, but is only the 9th longest of any highway classification in the state. Before the 1939 renumbering, SH 4 was the longest state highway.