US HIGHWAY NUMBERS
The numbered US highway system was adopted during the 1920's, as automobiles became more affordable and more roads became paved. Odd numbered highways generally would run north and south, with higher numbers to the west. And even numbered highways generally would run east and west, with higher numbers to the south. Three digit highways are usually variations or branches of the highways with those last two digits. One exception is US 101, along the Pacific Coast, which really is part of the two digit numbering system.
Link to an interesting Web site, which provides an excellent background on US highways.
In 1956, the Interstate highway system was adopted, and some of the US highways eventually lost their identity. Most notably the most famous US highway, Route 66, which is no longer an official US highway. Some US highways became state highways, with the same or different highway numbers.
One significant renumbering of US highways occurred in Michigan, in 1961. As Interstate 94 was completed between Detroit and New Buffalo, US 12, existing over that corridor, lost its designation. And US 112 was renumbered US 12.