Map of US highway 78 and family, and US 74
US 74 is represented by the purple line beginning in Wilmington NC. Its current western terminus is near Cleveland TN (according to signage). Historically the route continued into Chattanooga, and some maps suggest that Tennessee transportation officials may still consider it to share pavement with I-75 into that city.
US 78 is the purple line from Charleston SC to Memphis TN.
US 178 is the blue line from Dorchester SC to Rosman NC.
US 278 is the blue line from Hilton Head Island SC to Wickes AR.
US 378 is the blue line from Conway SC to Washington GA.