posted 06-16-2002 07:15 PM         

INTERSTATE HIGHWAYS: Under the plan developed for a national interstate highway system during the Eisenhower administration, interstate highways (marked with the familiar red-and-blue shields) are numbered according to the following rules:
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.
NORTH-SOUTH EXAMPLES: Interstate 5 runs through California, Oregon, and Washington, from the Mexican border to the Canadian border. Interstate 95 traverses the east coast from the Canadian border near Houlton, Maine, to Florida's southern tip in Miami.
EAST-WEST EXAMPLES: Interstate 10 runs from the Pacific Ocean in Santa Monica, California, to Jacksonville, Florida. Interstate 90 crosses the USA from Seattle, Washington, to Boston, Massachusetts.
(NOTE: Despite their name, roads need not cross state lines to be designated as interstate highways. Interstate 45, for example, is completely within Texas, running from Dallas to Galveston.)
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 I-710 freeway is a spur branching off Interstate 10 in 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 5 near the city of San Fernando and arcs through western Los Angeles and Orange counties before rejoining Interstate 5 in Irvine.
(NOTE: Three-digit interstate highway numbers are not unique. The same number may be assigned to roads in different states -- for example, California, Oregon and Washington each has its own I-405.)
And, just to make things even more confusing, there are quirky exceptions to these rules:
I-238 is not a spur off Interstate 38, because Interstate 38 doesn't exist.
Interstate 99 was assigned a high two-digit number (through legislation), even though it is not a major highway (it runs for only 58 miles between Wolfsburg and Bald Eagle in Pennsylvania) and is farther west than other interstates with lower numbers.
U.S. HIGHWAYS: U.S. Highways (or Routes), marked with black-and-white badge-shaped signs, are assigned one- to three-digit numbers. Like interstate highways, they designate north-south roads with odd numbers and east-west roads with even numbers, but the numbering scheme is inverted: the numbers of north-south routes grow larger from east to west, and the numbers of east-west routes grow larger from north to south.
NORTH-SOUTH EXAMPLES: US Highway 1 runs from Fort Kent, Maine, to Key West, Florida. US Highway 99 used to run from the Canadian border at Blaine, Washington, almost to the Mexican border at the city of Calexico, California. (Highway 99 has since been supplanted by Interstate 5 and is no longer designated as a U.S. Highway.)
EAST-WEST EXAMPLES: US Highway 14 runs from Chicago, Illinois to Yellowstone National Park in Wyoming. US Highway 82 runs from Alamogordo, New Mexico, to Brunswick, Georgia.
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 Los Angeles basin through the high desert, yet it's the 14 Freeway.
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 letter H rather than the standard I, however.