This section of American Roads is about United State Highways. In America, this refers to the older system of highways that have black and white shield signs. In other words, the limited access, multi-lane interstate freeways are not discussed. There are a few excellent web sites for those interested in interstate freeways, such as interstate-guide.com.
The American national highway system that uses numbers rather than names, developed from the named auto trails. Good roads were created across America, and the idea of interstate highways was born. In the mid 1920s, federal and state highway officials met to create an official national highway system.
State highway officials formed an organization in 1914 known as the American Association of State Highway Officials (AASHO). This organization proposed that the federal government work with state governments to create an official interstate highway system. Secretary of Agriculture, Howard Gore, appointed a joint board of state and federal road officials. A system of highways was mapped out in 1925, and a sub-committee worked on assigning numbers to them using a logical pattern. The new Secretary of Agriculture, J. M. Jardine, approved the system on November 18, 1925. The U.S. highway system included nearly 100000 miles of highways at the time of approval.
The numbering system for U.S. highways was part of the original plan. Even numbered highways go east and west, starting with the smallest numbers in the north (U.S. Route 2), and going to the largest double digit numbers in the southern U.S. (Route 96, and later Route 98). Odd numbered highways run north and south, starting in the east (U.S. Route 1) and going to the largest numbers in the west (Route 101).
The primary east-west routes end in a 0, while the main north-south highways end in 1.
Branch highways were given three-digit numbers based on the parent highway. For example, U.S highways 166 and 266 branch off of Route 66 (in Kansas and Oklahoma respectively).
There are some exceptions to these rules that developed as time passed.
The original national highway system still exists. However, many of the original U.S. highways were partially or fully decommissioned after the creation of the Eisenhower Interstate Highway System in 1956. This is especially true in the western U.S. The roadways for many of these decommissioned highways can still be driven. Some have become state or county highways, while others have become no more than local country roads. Starting in the 1990's, some former U.S. highways have been signed and officially recognized as historic routes.
The Wikipedia site has a nearly complete list of U.S. Highways, and many of them have articles as part of the U.S. Highways WikiProject. This site will concentrate on a few of the more important interstate and transcontinental U.S. highways, especially those that developed from famous named auto trails.