Map of US highway 23 and family, with US 25 and US 121

US 23 is represented by the purple line from Mackinaw City MI to Jacksonville FL.

US 123 [I] (historic) is the red line from Jenkins to Pineville KY (this was once the southernmost segment of US 23, until that highway was rerouted southward out of Jenkins).

US 123 is the blue line from Greenville SC to near Toccoa GA.

US 223 is the blue line beginning in Somerset MI. Currently its southern end is in Sylvania OH (outside Toledo); the red line east from there to Toledo is the historic extent of US 223.

US 121 is also shown on this map (the green line from Beckley WV to Pound VA). Its number is a violation of US route numbering conventions, because it doesn't connect with US 21 (its implied parent route - see map). So I included it on this map to demonstrate how the highway would fit into the US 23 family, if it were to be assigned a number US x23. I think an even better option would've been US x19.

US 25 is the purple line beginning in Brunswick GA. The route splits into US 25E and US 25W in part of Kentucky and Tennessee. Currently its northern end is in Cincinnati OH; the red line north from there to Port Austin MI is the historic extent of US 25.