Establishment of Federal Highways
In 1925, the Federal Highway System was created to codify the numerous highways that had been built and were in the process of being built across the country. Funds were given to states to build, improve and maintain roads. In addition, national uniform engineering standards were established and uniform route numbers and signage came into use.
Routes running east and west were assigned even numbers, with the nine most important coast-to-coast routes given numbers ending in zero. North-South routes were given odd numbers with most important ones ending in five. East-West numbers started in the north with US-2 and went south. North-South roads were numbered from the east with US-1.
In North Carolina, the primary U.S. highways are numbers 1, 13, 15, 17, 19, 21 25, 29, 52, 64, 70 and 74. The -01 and -21 series were designed to be feeders or alternates of US-1 and US-21.
For more than 30 years, US-421 (the road I have driven from end-to-end, passed diagonally through the state was the main road connecting North Carolina's most populous cities: Boone, North Wilkesboro, Winston-Salem, Greensboro, Sanford, Dunn and Wilmington.
When It's Really Odd. --RoadDog