The highway system of the United States consists of US routes and interstates. In addition, all fifty states, the District of Columbia, Puerto Rico, Guam, and the U.S. Virgin Islands all maintain their own systems. Despite their names, US routes and interstates are the responsibility of the state department of transportation where they are located.
The US Routes (also known as U.S. Highways) are even numbered for east-west routes (with the lowest numbers along Canada) and are odd numbered for north-south routes (with the lowest numbers along the Atlantic Ocean). Three-digit highways, also known as "child routes," are branches off their main one- or two-digit "parents" (for example, U.S. Route 271 is a branch of U.S. Route 71). However, U.S. 101, rather than a "child" of U.S. 1, is considered a "mainline" U.S. route.
Interstates are also even numbered for east-west routes (but the lowest numbers are along Mexico and the Gulf of Mexico), and the odd-numbered routes are north-south routes (with the lowest numbers along the Pacific Ocean). Three-digit interstates are, generally, either beltways or spurs of their parent interstates (for example, Interstate 270 is a beltway around the city of Columbus, Ohio, and is connected to Interstate 70).
Each state and territory has its own system for numbering highways, some more systematic than others. Each state also has its own design for its highway markers; the number in a circle is the default sign, but many choose a different design somehow connected to their state. Many use an outline of the state with the number inside.
Approximately 10,000 articles have been written on interstate, US, and state roads.
