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Interstate Highways in the 48 contiguous states.
Purple routes are currently built and open freeways, blue are currently open spur routes, and green indicates proposed routes, future roads, or those currently under construction.

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 Statesmarker that is named for President Dwight D. Eisenhower, who championed its formation. The entire system, , has a total length of , 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 National Highway System.

While Interstate Highways usually receive substantial federal funding (90% federal and 10% state) and 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 95/495), was maintained by the federal government; its new span is now jointly owned and maintained by the states of Marylandmarker and Virginiamarker. There are also other Interstate Highways within the District of Columbiamarker, which is federal territory.

This freeway 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) use the Interstate system.


1955 Map: The planned status of U.S. highways in 1955, as a result of the developing Interstate Highway 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 as a young Army officer crossing the country in the 1919 Army Convoy on the Lincoln Highway, the first road across America. Eisenhower also had gained an appreciation of the Germanmarker Autobahn network as a necessary component of a national defense system while he was serving as Supreme Commander of the Allied forces in Europe during World War II. 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.

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. In 1939, BPR Division of Information chief Herbert S. Fairbank wrote a report entitled Toll Roads and Free Roads, "the first formal description of what became the interstate highway system," and in 1944 the similarly-themed Interregional Highways. The publication in 1955 of the General Location of National System of Interstate Highways, informally known as the Yellow Book, mapped out what became the Interstate System.

Although construction on the Interstate Highway System continues, I-70 through Glenwood Canyonmarker (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, such as Interstate 485 in North Carolinamarker. A few main routes not part of the original plan remain under construction, such as Interstate 22 in Tennesseemarker, Mississippimarker, and Alabamamarker, and the extension of Interstate 69 from Indianamarker to Texasmarker. Officials have also identified some non-Interstate corridors for future inclusion into the system, either by construction of new Interstate routes or upgrade of existing roads to Interstate standards.

Due to the cancellation of the Somerset Freeway, Interstate 95 is discontinuous in New Jerseymarker. Authorized by the federal government in 2004, the Pennsylvania Turnpike/Interstate 95 Interchange Projectmarker is scheduled to connect the separate sections of I-95 to form a continuous route, completing the final section of the original plan.

Three states have claimed the title of first Interstate Highway. Missouri claims that the first three contracts under the new program were signed in Missouri on August 2, 1956. The first contract signed was for U.S. 66. On August 13, 1956, Missouri awarded the first contract based on new Interstate Highway funding.

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 now designated I-70 and I-76 opened between Irwinmarker and Carlislemarker. The Commonwealth of Pennsylvania refers to the turnpike as "The Granddaddy of the Pikes".

Nebraska was the first state to complete its mainline Interstate Highway. The portion of Interstate 80 in Nebraska was completed on October 19, 1974.


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).

Speed limits

Being freeways, Interstate Highways usually have the highest speed limits in a given area. Speed limits are determined by individual states. From 1974 to 1987, the maximum speed limit on any highway in the United States was , in accordance with federal law. Currently, rural speed limits generally range from , although several portions of I-10 and I-20 in rural western Texas, along with a portion of I-15 in rural central Utah, have speed limits of . Typically, lower limits are established in the more densely populated Northeastern states, while higher speed limits are established in the less densely populated Southern and Western states.For example, some stretches of I-76 through Philadelphiamarker have a speed limit of .

Other uses

As one of the components of the National Highway System, Interstate Highways improve the mobility of military troops to and from airports, seaports, rail terminals, and other military bases. Interstate Highways also connect to other roads that are a part of the Strategic Highway Network, a system of roads identified as critical to the U.S.marker Department of Defensemarker.

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, Georgiamarker, and Charleston, South Carolinamarker, 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, Floridamarker 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 (to keep traffic flowing instead of having drivers stop for directions), and improving the dissemination of public information. As a result, the 2005 evacuation of New Orleans, Louisianamarker prior to hurricane Katrina 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.

Numbering system

Primary (1- and 2-digit) routes

The numbering scheme for the Interstate Highway System was developed in 1957 by the American Association of State Highway and Transportation Officials (AASHTO). The association's present 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.

In the numbering scheme, west–east highways are assigned even numbers and south–north 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 Canadamarker and Mexicomarker along the West Coast to I-95 between Canadamarker and Miamimarker along the east coast. Major west–east arterial Interstates increase in number from I-10 between Santa Monica, Californiamarker and Jacksonville, Floridamarker to I-90 between Seattle, Washingtonmarker and Boston, Massachusettsmarker with two exceptions. There is no Interstate 50 or Interstate 60, as routes with those numbers would likely pass through states which currently have U.S. Highway with the same numbers, which is not allowed under highway administration guidelines. Two-digit Interstates in Hawaiimarker, 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 (I-76, I-84, I-86, and I-88). 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 the Twin Citiesmarker of Minneapolismarker and Saint Paul, Minnesotamarker and the Dallas-Fort Worth Metroplex in Texasmarker.

AASHTO policy allows dual numbering so as to provide continuity between major control points. This is referred to as a concurrency or overlap. For example, I-75 and I-85 share the same roadway in Atlantamarker; this section, called the Downtown Connector, 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". In rare instances, two routes sharing the same roadway are signed as traveling in opposite directions; one such wrong-way concurrency is found between Wythevillemarker and Fort Chiswell, Virginiamarker where I-81 north and I-77 south are equivalent, as are I-81 south and I-77 north.

Auxiliary (3-digit) 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. 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 west/east 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.

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.

Alaska, Hawaii, and Puerto Rico

The Interstate Highway System also extends to Alaska, Hawaii, and Puerto Rico. Those in Hawaiimarker, all on the populous island of Oahumarker, carry the designation of H-x and connect military bases, though they are open to public use. Both Alaskamarker and Puerto Rico have public roads that receive funding from the Interstate program, although these routes are not signed as Interstate Highways (except on paper). These roads are neither planned for, nor built to, official Interstate Highway standards.


While the name implies that Interstate Highways cross state lines, many do not (for details, see List of intrastate Interstate Highways). Rather, they are funded federally with money shared among the states. The H-x Interstates in Hawaiimarker and the paper interstates in Alaska and Puerto Rico are funded in the same way as in the other states.

About 70% of the construction and maintenance costs of highways in the U.S. are covered through user fees (net of collection costs), primarily gasoline taxes collected by the federal government and state and local governments, and to a much lesser extent tolls collected on toll roads and bridges. The rest of the costs are borne by general fund receipts, bond issues, and designated property and other taxes. The federal contribution is overwhelmingly from motor vehicle and fuel taxes (93.5% in 2007), as is about 60% of the state contribution. However, local contributions are overwhelmingly from sources other than user fees. The portion of the user fees spent on highways themselves covers about 57% of costs, as approximately one-sixth of the user fees are diverted to other programs, prominently including mass transit. 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.

Parts of the system may 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 Diegomarker, Salt Lake Citymarker, Minneapolismarker, Houstonmarker, Denvermarker, Dallasmarker, Atlantamarker, and Washington, D.C.marker At present, federal law does not allow for a state to change a freeway section to a tolled section for all traffic.

Toll Interstate Highways

Approximately of toll roads are included in the Interstate Highway System. While federal legislation banned the collection of tolls on Interstates, many of the toll roads on the system were either completed or under construction when the Interstate Highway System was established. Since these highways provided logical connections to other parts of the system, they were designated as Interstate highways. Toll roads designated as Interstate highways were typically allowed to continue collecting tolls, but are generally ineligible to receive federal funds for maintenance and improvements.

Local maintenance

A few Interstates are maintained by local authorities:

Chargeable and non-chargeable Interstate routes

Interstate Highways financed with federal funds are known as "chargeable" Interstate routes, and are considered part of the network of highways. Federal laws allow highways funded similarly to state and U.S. highways to be signed as Interstates, if they meet the Interstate Highway standards and are logical additions or connections to the system.

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.


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 Manual on Uniform Traffic Control Devices (MUTCD). However, there are many local and regional variations in signage.

For many years, Californiamarker 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 Arizonamarker, 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 Yonkersmarker traveling north, and then west from Albany. On the Jane Addams Memorial Tollway, mileage markers count up from Chicago-O'Hare International Airportmarker traveling west, which is the starting point of the tollway.

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.

Interstate shield

Interstate Highways are signed by a number placed on a trademarked red, white and blue sign. A sample sign is 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 in which 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. The green shield is employed to mark the main route through a city's central business district, which intersects the associated Interstate highway at one (spur) or both (loop) ends of the business route. The route usually traverses the main thoroughfare(s) of the city's downtown area or other major business district. A city may have more than one Interstate-derived business route, depending on the number of Interstates passing through a city and the number of significant business districts therein.

Over time, the design of the Interstate shield has changed. In 1958, the Interstate shield designed by Texas Highway Department employee Richard Oliver was introduced, the winner of a contest which included 100 entries; at the time, 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 had nearly quadrupled, with some Interstate shields reaching in width.


By traffic volume

By direction

In elevation

In length

In width

  • Widest (as acknowledged in writing by FHWA as of 2004): 15 lanes: I-75 north of Atlanta, near the interchange with I-285 (as of 2004).
  • Widest (most grade-separated access-controlled through lanes): 21 lanes: I-5 along a 2-mile section between I-805 and SR-56, which was completed in April 2007.
  • Widest (most through lanes on continuous right-of-way): 26 lanes: I-10 in some sections in Houstonmarker , including frontage roads, general use, and HOV lanes. Though the frontage lanes form part of a single continuous right-of-way along with the inner general use and HOV lanes, the frontage roads themselves are not built to freeway standards as they lack full access control. Like many such frontage roads in Texas, they have driveways to adjacent properties (with "Katy Freeway" street addresses) and at-grade intersections.
  • Narrowest: 2 lanes: I-93 in New Hampshiremarker where it runs through Franconia Notch State Parkmarker, and I-81 on the southern span of the Thousand Islands Bridgemarker, are the only instances of a two lane highway – also called a Super-2 parkway – on the Interstate system, although some 2-lane International crossings over water are cited as the political end of some Interstates such as I-75.

By states and cities

See also


  1. See Interstate 295 and Interstate 395.
  2. McNichol, Dan. The Roads that Built America: The Incredible Story of the U.S. Interstate System. New York: Sterling Publishing Co., Inc., 2006. ISBN 1-4027-3468-9
  3. " Interregional Highways" (various scans). Also includes scans from Toll Roads and Free Roads as reprinted in Interregional Highways.
  4. Proposed I-41 in Wisconsin and partly-completed I-74 in North Carolina respectively are possible and current exceptions not adhering to the guideline. It is not known if the U.S. Highways with the same numbers will be retained in the states upon completion of the Interstate routes.
  5. "Paper" refers to Interstates which are funded under the same legislation as signed Interstates but are not signed with Interstate shields.
  6. Field, David. "On 40th birthday, interstates face expensive midlife crisis." Insight on the News, 29 July 1996, 40-42.
  7. Highway History, Federal Highway Administration, Accessed May 18, 2009
  8. New York State Department of Transportation - Region 11 (New York City) Built and Unbuilt Arterial System
  9. California Highways: Interstate Highway Types and the History of California's Interstates
  10. Manual on Uniform Traffic Control Devices, Introduction. Trademark has Serial Number 72239199, Registration Number 0835635.
  11. Interstate Shield Galleries
  12. Index of Interstate Business Loops
  13. " Ties to Texas" Texas Transportation Researcher newsletter, Volume 41, Number 4 (2005), Texas Transportation Institute.
  14. The Interstate is 50: Image Gallery. AASHTO.
  15. Most Travelled Urban Highways Average AADT
  16. Interstate 95 Annual Average Daily Traffic (AADT)
  19. "Interstate System Facts" retrieved 2009-08-15
  20. "Miscellaneous Interstate System Facts" retrieved 2009-08-15
  21. Steve Schmidt, " Four new southbound lanes at I-5/805 merge set to open", San Diego Union-Tribune, 28 March 2007, page number unknown.
  22. This fact is mentioned in Fresno,_California#Highways without any source.

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