Interstate 77 (I-77) is a north–south Interstate Highway in the Eastern United States. It traverses diverse terrain, from the mountainous state of West Virginia to the rolling farmlands of North Carolina and Ohio. It largely supplants the old U.S. Route 21 (US 21) between Cleveland, Ohio, and Columbia, South Carolina, as an important north–south corridor through the middle Appalachian Mountains. The southern terminus of I-77 is in Cayce, South Carolina, in Lexington County at the junction with I-26. The northern terminus is in Cleveland at the junction with I-90. Other major cities that I-77 connects to include Columbia, South Carolina; Charlotte, North Carolina; Charleston, West Virginia; and Akron, Ohio. The East River Mountain Tunnel, connecting Virginia and West Virginia, is one of only two instances in the U.S. where a mountain road tunnel crosses a state line. The other is the Cumberland Gap Tunnel, connecting Tennessee and Kentucky. I-77 is a snowbird route to the Southern U.S. for those traveling from the Great Lakes region.

U.S. Route 21 or U.S. Highway 21 (US 21) is a major north–south United States Numbered Highway in the Southeastern United States that extends 394 miles (634 km). The southern terminus is in Hunting Island State Park, South Carolina, 14.4 miles (23.2 km) south of the junction with US 21 Business and South Carolina Highway 802 (SC 802) in Beaufort. The northern terminus is in Wytheville, Virginia, at an interchange with Interstate 81 (I-81) and US 52. Despite the "1" indicating that it is a major north–south highway, US 21 only travels through three states and is no longer a cross-country route as it has been replaced with I-77 in both Ohio and West Virginia. It also has the second-shortest length of all of the major north–south routes, just behind US 91. The route travels through the states of South Carolina, North Carolina, and Virginia. It also connects through major Southeastern cities such as Columbia, South Carolina, and Charlotte, North Carolina. The northern portion of the road travels parallel to I-77 in northern South Carolina and North Carolina. The road also has three interchanges with I-26 in Lexington and Calhoun counties in South Carolina.