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 (also called freeways or expressways) in the United States that is named for the President who championed its creation. The Interstate Highway System is a separate system within the larger National Highway System. The entire system, , has a total length of 46,837 miles (75,376 km), making it both the largest highway system in the world and the largest public works project in history.
While Interstate highways usually receive substantial federal funding (90% Federal and 10% state), and they comply with Federal standards, they are owned, built, and operated by the states or toll authorities. For example, the original Woodrow Wilson Bridge (part of Interstate 95 and Interstate 495), was maintained by the Federal government; its new span is now jointly owned and maintained by the State of Maryland and the Commonwealth of Virginia. There are also other Interstate highways within the District of Columbia, which is Federal territory. See Interstate 295 and Interstate 395.
This expressway system serves nearly all major U.S. cities, with many Interstates passing through downtown areas. The distribution of virtually all goods and services involves Interstate highways at some point. Residents of American cities commonly use urban Interstates to travel to their places of work. The vast majority of long-distance travel, whether for vacation or business, uses the national road network; of these trips, about one-third (by the total number of miles driven in the country in 2003) utilize the Interstate system.
The Interstate Highway System was authorized by the Federal-Aid Highway Act of 1956 – popularly known as the National Interstate and Defense Highways Act of 1956 – on June 29. It had been lobbied for by major U.S. automobile manufacturers and championed by President Dwight D. Eisenhower, who was influenced by his experiences in 1919 as a young Army officer crossing the country in a truck convoy (following the route of the Lincoln Highway), and his appreciation of the German Autobahn network as a necessary component of a national defense system. In addition to facilitating private and commercial transportation, it would provide key ground transport routes for military supplies and troop deployments in case of an emergency or foreign invasion. (Memories were still strong of reported Japanese threats to invade the West Coast of the U.S. during WW II.)
Initial federal planning for a nationwide highway system began in 1921, when the Bureau of Public Roads asked the Army to provide a list of roads it considered necessary for national defense. This resulted in the Pershing Map. Later that decade, highways such as the New York parkway system were built as part of local or state highway systems. As automobile traffic increased, planners saw a need for such an interconnected national system to supplement the existing, largely non-freeway, United States Numbered Highway system. By the late 1930s, planning had expanded to a system of new superhighways. In 1938, President Franklin D. Roosevelt gave BPR chief Thomas MacDonald a hand-drawn map of the U.S. marked with eight superhighway corridors for study. The publication General Location of National System of Interstate Highways maps out what became the Interstate System, and is informally known as the Yellow Book.
Although construction on the Interstate Highway System actually continues, I-70 through Glenwood Canyon (completed in 1992) is often cited as the completion of the originally-planned system. The initial cost estimate for the system was $25 billion over 12 years; it ended up costing $114 billion (adjusted for inflation, $425 billion in 2006 dollars) and taking 35 years to complete. Additional spurs and loops/bypasses remain under construction. See Interstate 485 in North Carolina for an example. Also, a few additonal main routes remain under construction. See Interstate 22 in Alabama and Tennessee.
Due to the cancellation of the Somerset Freeway, Interstate 95 is discontinuous in New Jersey. When the Pennsylvania Turnpike/Interstate 95 Interchange Project concludes in or around 2009, the last section of the original plan will be completed.
Kansas claims that it was the first to start paving after the act was signed. Preliminary construction had taken place before the act was signed, and paving started September 26, 1956. The state marked its portion of I-70 as the "first project in the United States completed under the provisions of the new Federal-Aid Highway Act of 1956."
According to information liaison specialist Richard Weingroff, the Pennsylvania Turnpike could also be considered one of the first Interstate highways. On October 1, 1940, of the highway opened between Irwin and Carlisle. The state of Pennsylvania refers to the turnpike as "The Granddaddy of the Pikes".
The American Association of State Highway and Transportation Officials (AASHTO) has defined a set of standards that all new Interstates must meet unless a waiver from the Federal Highway Administration (FHWA) is obtained. One almost absolute standard is the controlled access nature of the roads. With few exceptions, traffic lights (and cross traffic in general) are limited to toll booths and ramp meters (metered flow control for lane merging during rush hour).
Interstate highways usually have the highest speed limits in a given area. Speed limits are determined by individual states. Rural speed limits generally range from 65 to 75 miles per hour (105–120 km/h), although remote portions of I-10 and I-20 in rural western Texas have speed limits of . Typically, lower limits are established in the more densely populated Northeastern states and urban areas of the Midwest, while higher speed limits are established in the less densely populated Southern and Western states.
The system has also been used to facilitate evacuations in the face of hurricanes and other natural disasters. An option for maximizing traffic throughput on a highway is to reverse the flow of traffic on one side of a divider so that all lanes become outbound lanes. This procedure, known as contraflow lane reversal, has been employed several times for hurricane evacuations. After public outcry regarding the inefficiency of evacuating from southern Louisiana prior to Hurricane Georges' landfall in September 1998, government officials looked towards contraflow to improve evacuation times. In Savannah, Georgia and Charleston, South Carolina in 1999, lanes of Interstates 16 and 26 were used in a contraflow configuration in anticipation of Hurricane Floyd, with mixed results.
In 2004, contraflow was employed ahead of Hurricane Charley in the Tampa, Florida area and on the Gulf Coast before the landfall of Hurricane Ivan; however, evacuation times there were no better than previous evacuation operations. Engineers began to apply lessons learned from the analysis of prior contraflow operations, including limiting exits, removing troopers (so as to keep traffic flowing, instead of having drivers stop for directions), and improving the dissemination of public information. As a result, the 2005 evacuations of New Orleans, Louisiana, and Houston, Texas prior to hurricanes Katrina and Rita ran far more "smoothly".
A widespread urban legend states that one out of every five miles of the Interstate Highway System must be built straight and flat so as to be usable by aircraft during times of war. Contrary to popular lore, Interstate highways are not designed to serve as airstrips.
The numbering scheme for the Interstate Highway System (as well as the U.S. Highway System) was developed in 1957 by the American Association of State Highway and Transportation Officials (AASHTO). The association's latest numbering policy dates back to August 10, 1973. Within the continental United States, primary Interstates – also called main line Interstates or two-digit Interstates – are assigned numbers less than 100. AASHTO policy discourages Interstate and U.S. highways having the same number within the same state, although there are a few exceptions.
Within this category, east–west highways are assigned even numbers, and north–south highways are assigned odd numbers. Odd route numbers increase from west to east, and even-numbered routes increase from south to north, though there are exceptions to both principles in several locations. Numbers divisible by 5 are intended to be major arteries among the primary routes, carrying traffic long distances. Major north–south arterial Interstates increase in number from I-5 between Canada and Mexico along the West Coast to I-95 between Miami and Canada along the east coast. Major east–west arterial Interstates increase in number from I-10 between Santa Monica, California and Jacksonville, Florida to I-90 between Seattle and Boston. Two-digit Interstates in Hawaii, as well as the "paper" Interstates of Alaska and Puerto Rico, are numbered sequentially in order of funding, without regard to the rules on odd and even numbers.
Several two-digit numbers are shared between two roads at opposite ends of the country. Some of these were the result of a change in the numbering system as a result of the new policy adopted in 1973. Previously, letter-suffixed numbers were used for long spurs off primary routes; for example, western I-84 was I-80N, as it went north from I-80. The new policy stated that "No new divided numbers (such as I-35W and I-35E, etc.) shall be adopted." The new policy also recommended that existing divided numbers be eliminated as quickly as possible; however, I-35W and I-35E still exist in Minnesota and Texas.
AASHTO policy allows dual numbering so as to provide continuity between major control points. This is usually referred to as a concurrency or overlap. For example, I-75 from Chattanooga, Tennessee to Macon, Georgia and I-85 from Greenville, South Carolina to Montgomery, Alabama share the same roadway (the Downtown Connector) in Atlanta. As a result, this section of Interstate Highway is labeled both I-75 and I-85. Concurrencies between Interstate and U.S. Route numbers are also allowed per AASHTO policy, so long as the length of the concurrency is "reasonable". The concurrency policy may lead to confusion when two routes on the same roadway are marked in opposite directions. This is called a wrong-way concurrency. An example of a wrong-way concurrency is the stretch of I-80 and I-580 in California between Emeryville and Richmond; I-80 east and I-580 west are equivalent, and vice versa.
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. See List of auxiliary Interstate Highways for examples.
In the example above, City A has an even-numbered circumferential highway. City B has an even-numbered circumferential beltway and an odd-numbered spur. City C has an even-numbered circumferential highway and an odd numbered spur. Because cities A, B and C are in the same state, each auxiliary route carries a distinct three-digit route number.
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 directions are used as a directional labeling system, as opposed to compass directions.
Virtually 100 percent of the construction and maintenance costs are funded through fuel taxes collected by states and the federal government, and tolls collected on toll roads and bridges. In the eastern United States, large sections of some Interstate highways planned or built prior to 1956 are operated as toll roads.
As American suburbs have expanded, the costs incurred in maintaining freeway infrastructure have also grown, leaving little in the way of funds for new Interstate construction. This has led to the proliferation of toll roads (turnpikes) as the new method of building limited-access highways in suburban areas. Some Interstates are privately maintained (e.g., VMS maintains I-35 in Texas) to meet rising costs of maintenance and allow state departments of transportation to focus on serving the fastest growing regions in their respective states.
It is possible that parts of the system will have to be tolled in the future to meet maintenance and expansion demands, as has been done with adding toll HOV/HOT lanes in cities such as San Diego, Salt Lake City, Minneapolis, Houston, Denver, Dallas, Atlanta, and Washington, D.C.
Called "non-chargeable" Interstate routes, these additions fall under two categories: routes that already meet Interstate standards, and routes not yet upgraded to Interstate standards. Only routes that meet Interstate standards may be signed as Interstates once their proposed number is approved.
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, Caltrans 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 I-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. Exits on the New York State Thruway count up from Yonkers traveling north, and then west from Albany. 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, 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.
Interstate highways are signed by a number placed on a trademarked red, white and blue sign as shown to the right. In the original design, the state was listed above the highway number, but in many states, this area is now left blank. The sign usually measures 36-in (91 cm) high, and is 36-in wide for two-digit Interstates or 45-in (114 cm) for three-digit Interstates.
Interstate business loops and spurs use a special shield where the red and blue are replaced with green, the word BUSINESS appears instead of INTERSTATE, and the word SPUR or LOOP usually appears above the number.
Over time, the design of the Interstate shield has changed. In 1958, when the Interstate shield was introduced, the shield color was a dark navy blue and only 17-in (41 cm) wide. The MUTCD standards revised the shield in the 1961, 1972, 1988, and 2000 editions. By 2000, the shield size nearly doubled, with some Interstate shields reaching in diameter.
The most extreme directional points of the Interstate Highway system are:
The highest point on the Interstate Highway System is at the Eisenhower Tunnel on Interstate 70 in the Rocky Mountains of Colorado, at the Continental Divide (elev. ). The lowest point on land is on Interstate 8 at the New River near Seeley, California (elev. ). The lowest point under water is on Interstate 95 in the Fort McHenry Tunnel under the Inner Harbor in Baltimore, Maryland (elev. ).
The longest Interstate highway is Interstate 90, which runs between Boston, Massachusetts and Seattle, Washington. The longest north-south Interstate highway is generally cited as Interstate 95; when completed, it will run between Miami, Florida and the Canadian border (there is a gap in New Jersey). The shortest (albeit unsigned) Interstate is Interstate 878, a portion of New York State Route 878 adjacent to John F. Kennedy International Airport in Queens, New York. Another short unsigned Interstate is Interstate 110 near downtown El Paso, with a length of . The shortest signed Interstate is Interstate 375 in downtown Detroit, Michigan, at .
The shortest Interstate route segment within a state (or federal district) is I-95 in the District of Columbia, which is long.