There are several different highway systems in use in the United States, including the federally-controlled interstates, the older US highways and individual state highways. Each system uses its own method to generate highway numbers, although there is usually a method to the madness. The highway numbers are intended to aid navigation by removing the ambiguity of street names which could change from state to state. If a driver understood how these numbers are assigned, he or she would know the general direction of an unfamiliar highway and the general area of the country it crossed.
Before the year 1925, there were very few if any official highway numbers. Individual roads or trails would be combined to form interstate routes with names such as the Lincoln Highway or the Dixie Overland Highway. These driving trails came complete with associations dedicated to their preservation and promotion, although some of these associations were formed for more commercial reasons. Drivers seeking to drive across the country on one of these trails often became confused whenever several different routes overlapped or used the same roads. Using names to designate interstate highways soon became too problematic for general tourism. In 1925, meetings were held to implement a new plan of uniform highway numbers in place of ambiguous trail names.
US highway numbers are assigned according to direction and purpose. Routes running east/west are given even numbers, while north/south routes are given odd numbers. The lowest numbers begin at the northern border near Canada and the eastern coast of Maine. Therefore, US Highway 1 is a north/south route originating in Maine and US Highway 2 is an east/west route that skirts the Canadian border. The numbers for the US highway system continue in a consecutive order, with the numbers increasing as they approach the west coast and the Gulf of Mexico.
Three digit highway numbers often denote spurs or branches from a main interstate route. One example of this is the San Francisco Bay Area. The primary east/west route through the area is I-80, but offshoots of this freeway include I-280, I-580, I-680, I-880, and I-980. A diversion around a major city might have a number "2" preceding the name of the main interstate. I-275, for example, is a circular interstate spur which bypasses Cincinnati, Ohio, and connects on both ends to I-75. Additionally, freeways may be numbered according to the roads that they link; I-805 in San Diego actually links east/west route I-8 with north/south route I-5 several miles east of where the two freeways intersect.
All highway numbers fall under the auspices of the American Association of State Highway and Transportation Officers (AASHTO), the organization which approved the first highway numbers in 1925. AASHTO works with other government agencies to designate new numbers according to their system. Individual states can also create numbers for new roads built within their borders, and are not always obligated to accept the numbers adopted by neighboring states. This can result in some confusion for drivers, as one numbered state highway can become an entirely different numbered highway in the next state. By unifying these roads under a single US Highway banner, the AASHTO hopes to eliminate much of the confusion.