Interstate Highway System

The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the Interstate Highway System (or simply, the Interstate System), is a network of limited-access highways that is named for President Dwight D. Eisenhower, who championed its creation. The entire system, has a total length of 46876 mi,; making it both the largest highway system in the world and the largest public works project in history. The Interstate Highway System is a subsystem of the US National Highway System.

The longest Interstate Highway is Interstate 90, which runs 3099 mi between Boston, Massachusetts and Seattle, Washington. The longest north-south Interstate Highway is generally cited as Interstate 95; when completed, it will run 1927 mi between Miami, Florida and the Canadian border (there is a gap in New Jersey). The shortest Interstate is Interstate 878, adjacent to John F. Kennedy International Airport in Queens, New York. Another short Interstate is Interstate 110 near downtown El Paso, with a length of 0.92 mi. The shortest two-digit Interstate Highway is Interstate 97, running from Baltimore to Annapolis in Maryland. This highway is 17.62 mi. The shortest Interstate route segment within a state (or federal district) is 0.11mi of I-95 in the District of Columbia, where it crosses the Potomac River on the Woodrow Wilson Bridge.

Auxiliary Interstates
Auxiliary Interstate Highways are circumferential, radial, or spur highways that principally serve urban areas. These types of Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of a nearby primary Interstate Highway. Spur routes deviate from their parent and do not return, with a few exceptions; these are given an odd first digit. Circumferential and radial loop routes return to Interstate Highways, and are given an even first digit. Due to the large number of these routes, auxiliary route numbers may be repeated in different states along the mainline. Some auxiliary highways do not follow these guidelines, however.

Unlike primary Interstates, three-digit Interstates are signed as either east/west or north/south, depending on the general orientation of the route, without regard to the route number. For some looped Interstate routes, inner/outer are used as a directional labeling system, as opposed to compass directions.

Business routes
AASHTO defines a category of special routes separate from primary and auxiliary Interstate designations. These routes do not have to comply to Interstate construction standards, but are routes that may be identified and approved by the association. The same route marking policy applies to both U.S. Numbered Highways and Interstate Highways; however, business route designations are sometimes used for Interstate Highways.

Known as Business Loops and Business Spurs, these routes that principally travel through the corporate limits of a city, passing through the central business district of the city. Business routes are used when the regular route is directed around the city.

Signage
The majority of Interstates have exit numbers. All traffic signs and lane markings on the Interstates are supposed to be designed in compliance with the MUTCD. However, there are many local and regional variations in signage.

For many years, California was the only state that did not use an exit numbering system. It was granted an exemption in the 1950s due to having an already largely completed and signed highway system; at the time, placing exit number signage across the state was deemed too expensive. Since 2002, however, California has begun to incorporate exit numbers on all its freeways – Interstate, U.S., and state routes alike. To mitigate costs, the California Department of Transportation commonly installs exit number signage only when a freeway or interchange is built, reconstructed, retrofitted, or repaired. The majority of the exits along California's Interstates now have exit number signage, particularly in rural areas.

In most states, the exit numbers correspond to the mileage markers on the Interstates. However, on Interstate 19 in Arizona, length is measured in kilometers instead of miles, in part because the road runs south to the Mexican border. On most even-numbered Interstates, mileage count increases from west to east; on odd-numbered Interstates, mileage count increases from south to north. Some tollways, including the New York State Thruway and Jane Addams Memorial Tollway, use radial exit numbering schemes. On the Jane Addams Memorial Tollway, mileage markers count up from Chicago-O'Hare International Airport traveling west.

Many northeastern states label exit numbers sequentially, regardless of how many miles have passed between exits. States in which Interstate exits are still numbered sequentially are Connecticut, Delaware, Massachusetts, New Hampshire, New York, Rhode Island and Vermont. Maine, Pennsylvania, Virginia, Georgia and Florida followed this system for a number of years, but recently converted to mileage-based exit numbers. The Pennsylvania Turnpike uses both mile marker numbers and sequential numbers. Mile marker numbers are used for signage, while sequential numbers are used for numbering interchanges internally. The New Jersey Turnpike also has sequential numbering, but other Interstates within New Jersey generally use mile markers.