The site here is the United States Department of Transportation which is involved in numbering highways.

United States Numbered Highways

The system of United States Numbered Highways (often called U.S. Routes or U.S. Highways) is an integrated system of roads and highways in the United States numbered within a nationwide grid. As these highways were coordinated among the states, they are infrequently referred to as Federal Highways, but they have always been maintained by state or local governments since their initial designation in 1926. The numbers and locations are coordinated by the American Association of State Highway and Transportation Officials (AASHTO). The only federal involvement in the AASHTO is a non-voting seat for the United States Department of Transportation. North-to-south highways are odd-numbered, with lowest numbers in the east and highest numbers in the west. Similarly, west-to-east highways are even-numbered, with the lowest numbers in the north and highest numbers in the south. Major north–south routes have numbers ending in "1" while major east–west routes have numbers ending in "0". Three-digit numbered highways are spur routes of each parent highway but are not necessarily connected to their parent route. Divided routes exist to provide two alignments to one route, even though many have been eliminated, while special routes, usually posted with a banner, can provide various routes, such as an alternate or bypass route, for a U.S. Highway. The Interstate Highway System has largely replaced the U.S. Highways for through traffic, though many important regional connections are still made by U.S. Highways and new routes are still being added.

• http://en.wikipedia.org/wiki/United States Numbered Highways