Origins: Most of us probably don't stop to ponder why the maze of U.S. interstates and highways we travel in our automobiles are numbered the way they are — just trying to find the correct road, get on it heading the right direction, and exit at the proper
- Interstate highways established under the aegis of the Federal Aid Highway Act of 1956.
- U.S. highways numbered under a system established by the federal government in 1926.
- Regional roads numbered under individual state and county systems.
- Major interstate highways are identified by one- or two-digit numbers. North-south routes are assigned odd numbers, with the numbers growing larger from west to east; east-west routes are assigned even numbers, with the numbers growing larger from south to north.
- Interstate routes that branch off major, long-distance routes are assigned three-digit numbers. The last two numbers indicate the parent route, and the first digit signifies the road's function (i.e., an odd digit for a spur running directly to a city; an even digit for a road that loops around a metropolitan area).
EXAMPLES: California's(NOTE: Three-digit interstate highway numbers are not unique. The same number may be assigned to roads in different states
I-710freeway is a spur branching off Interstate 10in Monterey Park and terminating in Long Beach. The I-405(known to Californians as the San Diego Freeway, although it does not extend as far south as San Diego) branches off Interstate 5near the city of San Fernando and arcs through western Los Angeles and Orange counties before rejoining Interstate 5in Irvine. — forexample, California, Oregon and Washington each has its own I-405.)
NORTH-SOUTH EXAMPLES:(NOTE: Despite their name, roads need not cross state lines to be designated as interstate highways.
Interstate 5runs through California, Oregon, and Washington, from the Mexican border to the Canadian border. Interstate 95traverses the east coast from the Canadian border near Houlton, Maine, to Florida's southern tip in Miami. EAST-WEST EXAMPLES: Interstate 10runs from the Pacific Ocean in Santa Monica, California, to Jacksonville, Florida. Interstate 90crosses the USA from Seattle, Washington, to Boston, Massachusetts.
I-238is not a spur off Interstate 38, because Interstate 38doesn't exist. (The curious can read Grant Cooper's lengthy explanation of how this came to be.)
Interstate 99was assigned a high two-digit number (through legislation), even though it is not a major highway (it runs for only 58 milesbetween Wolfsburg and Bald Eagle in Pennsylvania) and is farther west than other interstates with lower numbers.
NORTH-SOUTH EXAMPLES:STATE AND COUNTY ROUTES: Individual states and counties use different types of signage to designate regional state and county roads. For the most part, states follow the Interstate and U.S. Highway patterns of assigning odd numbers to north-south routes and even numbers to east-west routes, but as always there are exceptions. California's Antelope Valley Freeway runs almost due north from the
US Highway 1runs from Fort Kent, Maine, to Key West, Florida. US Highway 99used to run from the Canadian border at Blaine, Washington, almost to the Mexican border at the city of Calexico, California. ( Highway 99has since been supplanted by Interstate 5and is no longer designated as a U.S. Highway.)
US Highway 14runs from Chicago, Illinois to Yellowstone National Park in Wyoming. US Highway 82runs from Alamogordo, New Mexico, to Brunswick, Georgia.
In answer to that perennial trivia question, the state of Hawaii has three roads designated as interstate highways (all of them on the island of Oahu) because roads established under the purview of the Federal Aid Highway Act and receiving funding from the federal government are considered interstate highways, even if they fall completely within the borders of a single state. Hawaii's interstate highways are somewhat different than other interstates in that they are identified with numbers preceded by the
Last updated: 1 April 2011
Blocksma, Mary. Reading the Numbers. New York: Penguin Books, 1989 (pp. 96-99). Fausset, Richard. "Highway Numerology Muddled by Potholes in Logic." Los Angeles Times. 13 November 2001 (p. B2).