Interstate Highway System

Interstate Highway System
Interstate Highways in the 48 contiguous states. Dark orange routes are built and open freeways, blue are open auxiliary routes, and green dashed indicates proposed routes, future roads, or those under construction.
A rural stretch of Interstate 5 in California, with two lanes in each direction separated by a large grassy median, and with cross-traffic limited to overpasses and underpasses.

The Dwight D. Eisenhower National System of Interstate and Defense Highways, (commonly known as the Interstate Highway System, Interstate Freeway System or the Interstate), is a network of limited-access roads including freeways, highways, and expressways forming part of the National Highway System of the United States of America. The system, which is named for President Dwight D. Eisenhower, who championed its formation, serves nearly all major U.S. cities.[citation needed] Construction was authorized by the Federal-Aid Highway Act of 1956 and took 35 years. The network has since been extended and as of 2006 it had a total length of 46,876 miles (75,440 km).[1] About one-third of all miles driven in the country use the Interstate system (2003 figures).[2] The cost of construction has been estimated at $425 billion (in 2006 dollars), making it the largest public works project in history.[citation needed]




The Interstate Highway System 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.

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.[3] 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[who?] 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[by whom?] had expanded to a system of new superhighways.

In 1938, President Franklin D. Roosevelt gave Thomas MacDonald, chief at the Bureau of Public Roads, a hand-drawn map of the U.S. marked with eight superhighway corridors for study.[3] In 1939, Bureau of Public Roads 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.[4][5]

Eisenhower gained an appreciation of the German 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.[6] He recognized that the proposed system would also provide key ground transport routes for military supplies and troop deployments in case of an emergency or foreign invasion.

1955 map: The planned status of U.S. highways in 1965, as a result of the developing Interstate Highway System

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.[7] Assisting in the planning was Charles Erwin Wilson, who was still head of General Motors when President Eisenhower selected him as Secretary of Defense in January 1953.


The Interstate Highway System was authorized by the Federal Aid Highway Act of 1956[8] – popularly known as the National Interstate and Defense Highways Act of 1956 – on June 29.

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.[9]

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.[9]

A stretch of I-80 in Omaha, Nebraska with a typical Interstate reassurance sign with control city listed

According to information liaison specialist, Richard Weingroff, the Pennsylvania Turnpike could also be considered one of the first Interstate Highways. On October 1, 1940, 162 miles (261 km) of the highway now designated I-70 and I-76 opened between Irwin and Carlisle. The Commonwealth of Pennsylvania refers to the turnpike as the Granddaddy of the Pikes.[9]

Nebraska was the first state to complete its mainline Interstate Highway, finishing its section of Interstate 80 on October 19, 1974.[10] The opening of I-70 through Glenwood Canyon in 1992 is often cited as the completion of the originally planned system.[11][12] 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[13]) and took 35 years.[14]


Additional spurs and loops/bypasses remain under construction, such as Interstate 485 in North Carolina, which has been under construction since the 1980s. A few main routes not part of the original plan remain under construction, such as Interstate 22 in Tennessee, Mississippi, and Alabama and the extension of Interstate 69 from Indiana to Texas. 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 Jersey. Authorized by the federal government in 2004, the Pennsylvania Turnpike/Interstate 95 Interchange Project is scheduled to connect the separate sections of I-95 to form a continuous route, completing the final section of the original plan. Construction began in 2010.[15]


I-94 near Coloma, Michigan showing examples of Interstate standards: rumble safety strips on shoulders, pavement type of parallel grooved (tined) concrete, restricted overpass height signage, secondary road overpass signage in center median, newly FHWA instituted cable median barriers, and upcoming exit signage on right shoulder

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 55 miles per hour (89 km/h), in accordance with federal law.[16] Currently, rural speed limits generally range from 65 to 75 miles per hour (105 to 121 km/h), 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 80 mph (129 km/h). 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.[17] For example, the maximum speed limit is 65 mph (105 km/h) in New England, New York, and New Jersey, and 50 mph (80 km/h) in the District of Columbia.[17]

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. Department of Defense.[18]

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.[19]

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;[20] 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, Louisiana prior to hurricane Katrina ran far more smoothly.[21]

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.[22][23]

Numbering system

Primary (one- and two-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.[24] Within the continental United States, primary Interstates – also called main line Interstates or two-digit Interstates – are assigned numbers less than 100.[24]

In the numbering scheme, 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 five are intended to be major arteries among the primary routes, carrying traffic long distances.[3][25] Major north–south arterial Interstates increase in number from I-5 between Canada and Mexico along the West Coast to I-95 between Canada and Miami along the East Coast. Major west–east arterial Interstates increase in number from I-10 between Santa Monica, California and Jacksonville, Florida to I-90 between Seattle, Washington, and Boston, Massachusetts, 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. Highways with the same numbers, which is not allowed under highway administration guidelines.[24][26] Two-digit Interstates in Hawaii, as well as the "paper"[27] 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 due to a change in the numbering system as a result of a 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, "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, an I-35W and I-35E still exist in the Dallas-Fort Worth Metroplex in Texas, and an I-35W and I-35E that run through Minneapolis and Saint Paul, Minnesota still exist.[24]

AASHTO policy allows dual numbering to provide continuity between major control points.[24] This is referred to as a concurrency or overlap. For example, I-75 and I-85 share the same roadway in Atlanta; this 7.4-mile (11.9 km) section, called the Downtown Connector, is labeled both I-75 and I-85. Concurrencies between Interstate and U.S. Route numbers are also allowed in accordance with AASHTO policy, as long as the length of the concurrency is reasonable.[24] In rare instances, two routes sharing the same roadway are signed as traveling in opposite directions; one such wrong-way concurrency is found between Wytheville and Fort Chiswell, Virginia where I-81 north and I-77 south are equivalent (with that section of road traveling almost due east), as are I-81 south and I-77 north.

Auxiliary (three-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; 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.[28] Some auxiliary highways do not follow these guidelines, however. See List of auxiliary Interstate Highways for examples.

Examples of the auxiliary Interstate Highway numbering system

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.[28]

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.[29] 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.[29]

Alaska, Hawaii, and Puerto Rico

Anchor Point claims the distinction of being the most Westerly point on the contiguous highway system in North America
Map of routes in Puerto Rico that receive funding from the Interstate program, but are not signed as Interstate Highways

The Interstate Highway System also extends to Alaska, Hawaii, and Puerto Rico, even though these states have no direct land connections to other states. Those in Hawaii, all on the populous island of Oahu, carry the prefix H (e.g., Interstate H-1), connecting military bases as well as several communities around the island. Both Alaska and Puerto Rico have public roads that receive funding from the Interstate program, although these routes are signed with their local designation, not their Interstate Highway designation. These roads are neither planned for, nor built to, official Interstate Highway standards.[30]


I-787 in Watervliet, New York, showing the Exit 8 diamond interchange

In the contiguous United States, Interstate Highways are funded federally with money shared among the states. The H Interstates in Hawaii and the "paper" Interstates in Alaska and Puerto Rico are funded in the same way.

About 70% of the construction and maintenance costs of highways in the U.S. are covered through user fees (net of collection costs), primarily fuel 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 1956 Highway Trust Fund, established by the Highway Revenue Act, mandated a three-cent-per-gallon tax, soon increased to 4.5 cents. In 1993 the tax reached 18.4 cents per gallon where it remains.[31]

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.[32] 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.[33] 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)[34] 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 Diego, Salt Lake City, Minneapolis, Houston, Denver, Dallas, Atlanta, and Washington, D.C.. Although part of the tolling is an effect of the SAFETEA-LU act, which has put an emphasis on toll roads as a means to reduce congestion.[35][36] Present federal law does not allow for a state to change a freeway section to a tolled section for all traffic.[citation needed]

Toll Interstate Highways

An Interstate 376 trailblazer with the new black-on-yellow "Toll" sign.

Approximately 2,900 miles (4,700 km) of toll roads are included in the Interstate Highway System.[37] While federal legislation initially 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. Congress also decided that it was too costly to either build toll-free Interstates parallel to these toll roads, or directly repay all the bondholders who financed these facilities and remove the tolls.[38]

Toll facilities designated as Interstate highways (such as the Massachusetts Turnpike) were typically allowed to continue collecting tolls, but are generally ineligible to receive federal funds for maintenance and improvements. In addition, these toll facilities were grandfathered from Interstate Highway standards. A notable example is the western approach to the Benjamin Franklin Bridge in Philadelphia, where Interstate 676 has a surface street section through a historic area.

Policies on toll facilities and Interstate Highways have since changed. The Federal Highway Administration has allowed some states to collect tolls on existing Interstate Highways, while a recent extension of Interstate 376 included a section of Pennsylvania Route 60 that was tolled by the Pennsylvania Turnpike Commission before receiving Interstate designation. Also, newer toll facilities (like the tolled section of I-376, which was built in the early 1990s) must conform to Interstate standards. A new addition of the Manual on Uniform Traffic Control Devices in 2009 requires a black-on-yellow "Toll" sign to be placed above the Interstate trailblazer on Interstate Highways that collect tolls.[39]

Legislation passed in 2005 known as SAFETEA-LU, encouraged states to construct new Interstate Highways through "innovative financing" methods. SAFETEA-LU facilitated states to pursue innovative financing by easing the restrictions on building interstates as toll roads, either through state agencies or through public-private partnerships. However, SAFETEA-LU left in place a prohibition of installing tolls on existing toll-free Interstates, and states wishing to toll such routes to finance upgrades and repairs must first seek approval from Congress.

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 42,000-mile (68,000 km) network of highways. Federal laws[42] also allow non-chargeable Interstate routes, highways funded similarly to state and U.S. Highways to be signed as Interstates, if they both meet the Interstate Highway standards and are logical additions or connections to the system.[43] 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.


Commemorative sign introduced in 1993. Though the system was established during Dwight D. Eisenhower's presidency, the five stars commemorate his rank as General of the Army during World War II.

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). There are, however, 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; placing exit number signage across the state was deemed too expensive. California began to incorporate exit numbers on its freeways in 2002 – Interstate, U.S., and state routes alike. Caltrans commonly installs exit number signage only when a freeway or interchange is built, reconstructed, retrofitted, or repaired to control costs. Most exits along California's Interstates now have exit number signage, particularly in rural areas.[citation needed]

Exit numbers correspond to Interstate mileage markers in most states. on I-19 in Arizona, however, length is measured in kilometers instead of miles because, at the time of construction, a push for the United States to change to a metric system of measurement had gained enough traction that it was mistakenly assumed that all highway measurements would eventually be changed to metric; proximity to metric-using Mexico may also have been a factor, as I-19 indirectly connects I-10 to the Mexican Federal Highway system via surface streets in Nogales. Mileage count increases from west to east on most even-numbered Interstates; 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, which is the starting point of the tollway.

As of November 2010 the Illinois State Tollway Authority has redone the mileage markers to be uniform with the rest of the state on I-90 (Jane Addams Memorial/Northwest Tollway) and the I-94 section of the Tri-State Tollway, which previously had matched the I-294 section starting in the south at I-80/I-94/IL Route 394. The tollway is also currently in the process of adding exit number tabs to the exits.[citation needed]

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; as such, three of the main Interstate highways that remain completely within these states (89, 91, and 93) have interchanges numbered sequentially along their entire routes. Maine, Pennsylvania, Virginia, Georgia and Florida followed this system for a number of years, but since converted to mileage-based exit numbers. Georgia renumbered in 2000, while Maine did so in 2004. 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, including the portions which are signed as I-95 and I-78, also has sequential numbering, but other Interstates within New Jersey use mile markers.

I-87 in New York State is numbered in two sections. The section of I-87 that is a part of the New York State Thruway starts in Yonkers (exit 1) and continues north to Albany (exit 24). At Albany, the Thruway turns west and becomes I-90 for exits 25–61. From Albany north to the Canadian border, the exits on I-87 are numbered sequentially from 1 to 44 along the New York State Northway. This often leads to confusion as there is more than one exit on I-87 with the same number. For example, exit 4 on Thruway section of I-87 connects with the Cross County Parkway in Yonkers, but exit 4 on the Northway is the exit for the Albany airport. These two exits share a number but are located 125 miles (201 km) apart.

There are four common signage methods on Interstates: One is locating a sign on the ground to the side of the highway, mostly the right, and is used to denote exits, as well as rest areas, motorist services such as gas and lodging, recreational sites, and freeway names. Another method is attaching the sign to an overpass. The two most common methods involve overhead gantries; they can be mounted either on half-gantries that are located on one side of the highway, like the ground-mounted sign; or full gantries that bridge the entire width of the highway and often show two or more signs.

Interstate shield


Interstate Highways are signed by a number placed on a trademarked[3][44] red, white and blue sign. The sign for Interstate 24 is shown to the right. In the original design, the name of the state was displayed 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.[45]

Business Loop 24.svg

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.[46]

Several proposed Interstate shield design proposals submitted by the Texas Highway Department.

Over time, the design of the Interstate shield has changed. In 1957, the Interstate shield designed by Texas Highway Department employee Richard Oliver was introduced, the winner of a contest which included 100 entries;[47][48] at the time, the shield color was a dark navy blue and only 17-in (41 cm) wide.[45] The MUTCD standards revised the shield in the 1961, 1970, and 1978 editions.


  • Heaviest traveled: 390,000 vehicles per day: I-405 in Los Angeles, California (2006 estimate[49]).
  • Least traveled: 1,800 vehicles per day: I-95 just north of Houlton, Maine to the Canadian border (2001 estimate[50]).
  • Widest (most grade-separated access-controlled through lanes): 21 lanes—I-5 along a 2-mile (3.2 km) section between I-805 and SR-56 in San Diego, which was completed in April 2007.[56]
  • Widest (widest median): more than a mile—I-8 through the In-Ko-Pah grade in California at about a mile wide, I-84 through the Cabbage Hill grade in Oregon at 2 miles (3.2 km) wide, and I-24 through the Monteagle grade in Tennessee at 1.75 miles (2.82 km) wide.
  • Narrowest: 2 lanes—I-93 in New Hampshire where it runs through Franconia Notch State Park where it also lacks breakdown lanes and I-81 on the southern span of the Thousand Islands Bridge, are the only instances of a two lane highway – also called a Super-2 parkway – on the Interstate system.
  • Most states served by an Interstate: 15 states plus the District of Columbia: I-95 through FL, GA, SC, NC, VA, DC, MD, DE, PA, NJ, NY, CT, RI, MA, NH, and ME.[52]
  • Most Interstates in a state: 29 routes: New York, totaling 1,674.73 mi (2,695 km).[52]
  • Most Primary Interstates in a State: 12 routes: Illinois, Pennsylvania
  • Most Interstate mileage in a state: 3,233.45 mi (5,204 km): Texas, in 17 different routes.[52]

See also

Blank shield.svg U.S. Roads portal


  1. ^ "Interstate FAQ (Question #3)". Federal Highway Administration. 2006. Retrieved March 4, 2009. 
  2. ^ "Annual Vehicle Distance Traveled in Miles and Related Data". Federal Highway Administration. 2003. Retrieved December 21, 2006. 
  3. ^ a b c d McNichol, Dan (2006). The Roads that Built America: The Incredible Story of the U.S. Interstate System. New York: Sterling Publishing. ISBN 1-4027-3468-9. 
  4. ^ Weingroff, Richard F. (Summer 1996). "The Federal-State Partnership at Work: The Concept Man". Public Roads 60 (1). 
  5. ^ "Interregional Highways".  Also includes scans from Toll Roads and Free Roads as reprinted in Interregional Highways.
  6. ^ Petroski, Henry (2006). "On the Road". American Scientist 94 (5): 396–399. 
  7. ^ Norton, Peter (1996). "Fighting Traffic: U.S. Transportation Policy and Urban Congestion, 1955-1970". Essays in History (Corcoran Department of History at the University of Virginia). Retrieved 2008-01-17. 
  8. ^ "The Cracks are Showing" (Subcription required). The Economist. June 26, 2008. Retrieved October 23, 2008. [dead link]
  9. ^ a b c Weingroff, Richard F. (May 7, 2005). "Three States Claim First Interstate Highway". Federal Highway Administration. Retrieved February 16, 2008. 
  10. ^ Nebraska Department of Roads. "I-80 50th Anniversary Page". Retrieved August 23, 2009. 
  11. ^ "CDOT Fun Facts". Colorado Department of Transportation. Retrieved February 15, 2008. 
  12. ^ Stufflebeam Row, Karen; LaDow, Eva; Moler, Steve. "Glenwood Canyon 12 Years Later". Federal Highway Administration. 
  13. ^ Neuharth, Al (June 23, 2006). "Traveling Interstates is our Sixth Freedom". USA TODAY. 
  14. ^ Minnesota Department of Transportation (2006). "Mn/DOT celebrates Interstate Highway System's 50th Anniversary". Archived from the original on December 4, 2007. Retrieved January 17, 2008. 
  15. ^ "I-95/I-276 Interchange Project Meeting Design Management Summary — DRAFT: Design Advisory Committee Meeting #2" (PDF). Pennsylvania Turnpike Commission. 
  16. ^ "Nixon Approves Limit of 55 M.P.H." (Subscription required). New York Times: pp. 1, 24. January 3, 1974. Retrieved July 27, 2008. 
  17. ^ a b Carr, John (2007-10-11). "State traffic and speed laws". Retrieved January 10, 2008. 
  18. ^ Slater, Rodney E. (Spring 1996). "The National Highway System: A Commitment to America's Future". Retrieved january 10, 2008. 
  19. ^ Wolshon, PE, Brian (August 2001). ""One-Way-Out": Contraflow Freeway Operation for Hurricane Evacuation". Retrieved January 10, 2008. [dead link]
  20. ^ "Contraflow Implementation Experiences in the Southern Coastal States" (PDF). Florida Department of Transportation. Retrieved September 27, 2007. 
  21. ^ McNichol, Dan (December 2006). "Contra Productive". Roads & Bridges. Retrieved January 10, 2008. 
  22. ^ "Landing of Hope and Glory". Retrieved December 30, 2007. 
  23. ^ Weingroff, Richard F. (May–June 2000). "One Mile in Five: Debunking the Myth". Federal Highway Administration. Retrieved December 14, 2010. 
  24. ^ a b c d e f American Association of State Highway and Transportation Officials (January 2000). "Establishment of a Marking System of the Routes Comprising the National System of Interstate and Defense Highways" (PDF). Retrieved January 23, 2008. 
  25. ^ The Rambler (January 18, 2005). "Ask the Rambler: Was I-76 Numbered to Honor Philadelphia for Independence Day, 1776?". Federal Highway Administration. Retrieved January 17, 2008. 
  26. ^ "Interstate FAQ". Federal Highway Administration. Retrieved June 26, 2009. "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." 
  27. ^ Voss, Oscar (2007). "Interstate Ends Photos". Alaska Roads. Retrieved July 9, 2009. "'Paper' refers to Interstates that are funded under the same legislation as signed Interstates but are not signed with Interstate shields." 
  28. ^ a b Federal Highway Administration (March 22, 2007). "FHWA Route Log and Finder List". Retrieved January 23, 2008. 
  29. ^ a b American Association of State Highway and Transportation Officials (January 2000). "Establishment and Development of United States Numbered Highways" (PDF). Retrieved January 23, 2008. 
  30. ^ "Design: FHWA Route Log and Finder List - Additional Designations". Federal Highway Administration. Retrieved March 14, 2010. 
  31. ^ "When did the Federal Government begin collecting the gas tax?". Federal Highway Administration. 04/07/11. Retrieved 29 June 2011. 
  32. ^ "Highway Statistics 2007, Funding For Highways and Disposition of Highway-User Revenues, All Units of Government, 2007". Retrieved 2009-06-19. 
  33. ^ Field, David. (July 29, 1996). "On 40th birthday, Interstates Face Expensive Midlife Crisis". Insight on the News: pp. 40–42. 
  34. ^ "Projects by Type". VMS, Inc.. Retrieved January 10, 2008. 
  35. ^ Hart, Ariel. "1st Toll Project Proposed for I-20 East. Plan Would Add Lanes Outside I-285" (PDF). Atlanta Journal-Constitution. Retrieved September 27, 2007. 
  36. ^ VanMeter, Darryl D.. "Future of HOV in Atlanta" (PDF). Archived from the original on October 25, 2007. Retrieved September 27, 2007. 
  37. ^ "Highway History". Federal Highway Administration. Retrieved May 18, 2009. 
  38. ^ "Why Does The Interstate System Include Toll Facilities?". Federal Highway Administration. Retrieved April 26, 2011. 
  39. ^ "Standard Highway Signs and Markings (SHSM) Interim Releases for New and Revised Signs". Manual on Uniform Traffic Control Devices. Federal HIghway Administration. 
  40. ^ "Maryland's Interstate System" (PDF). Maryland State Highway Administration. 
  41. ^ "Region 11 (New York City) Built and Unbuilt Arterial System". New York State Department of Transportation. 
  42. ^ 23 U.S.C. §§ 1034
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  45. ^ a b "Interstate Shield Galleries". Interstate Guide. AARoads. 
  46. ^ "Index of Interstate Business Loops". Interstate Guide. AARoad. 
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External links

Main Interstate Highways (major interstates highlighted)
4 5 8 10 12 15 16 17 19 20 22 24 25 26 27 29 30
35 37 39 40 43 44 45 49 55 57 59 64 65 66 68 69
70 71 72 73 74 75 76 (W) 76 (E) 77 78 79 80 81 82
83 84 (W) 84 (E) 85 86 (W) 86 (E) 87 88 (W) 88 (E) 89 90
91 93 94 95 96 97 99 H-1 H-2 H-3
Unsigned  A-1 A-2 A-3 A-4 PRI-1 PRI-2 PRI-3
Lists  Primary 

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