Map of US highways 400 and 412
The numbers of these routes imply that they are branches of "US highway zero" and US 12, respectively. In reality they are not, and these numbers represent major violations of US route numbering conventions. Therefore I've shown these routes on the map below (instead of on the maps of their implied highway families).
US 400 is the green line from near Joplin MO to Granada CO. Some possibilities that make more sense than 400 are US x50, US x54, and US x66 (you can click to view maps of US 400 in the context of those families).
US 412 is the green line from Columbia TN to Springer NM. Some possibilities that make more sense than 412 are US x56, US x60, US x62, US x64, and US x70 (you can click to view maps of US 412 in the context of those families).