View source for Engineering:Interstate Highway System

{{Short description|Network of freeways in the United States}}
{{infobox state highway system
|title= Dwight D. Eisenhower National System of Interstate and Defense Highways
|shields= {{infobox road/shieldmain/USA|type=I|route=80}}{{infobox road/shieldmain/USA|type=BL|route=80}}[[File:MUTCD M1-10a.svg|70px|alt= Eisenhower Interstate System sign]]
|caption= Highway shields for Interstate&nbsp;80, Business Loop Interstate&nbsp;80, and the Eisenhower Interstate System
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| map_custom= yes
| map_notes= Primary Interstate Highways in the 48 contiguous states. Alaska, Hawaii, and Puerto Rico also have Interstate Highways.
|formed= {{start date and age|1956|06|29}}<ref name="FAHA56">{{cite magazine |last = Weingroff |first = Richard F. |title = Federal-Aid Highway Act of 1956, Creating the Interstate System |url = http://www.fhwa.dot.gov/publications/publicroads/96summer/p96su10.cfm |magazine = Public Roads |volume = 60 |issue = 1 |date = Summer 1996 |access-date = March 16, 2012 |issn = 0033-3735 |archive-url = https://web.archive.org/web/20120307133751/http://www.fhwa.dot.gov/publications/publicroads/96summer/p96su10.cfm |archive-date = March 7, 2012 |url-status = live }}</ref>
|length_mi= 48756
|length_ref= {{#tag:ref|{{As of|2020}}.<ref name="hm20" />|group=lower-alpha}}
|interstate= Interstate X (I-X)
|links= BL
}}
<!--{{Highway system OSM map
| highway_system_qid= Q94247
| frame-width       = 350
| frame-height      = 200
| frame-align       = right
| text              = Interactive map of the Interstate Highway System
}}-->

The '''Dwight D. Eisenhower National System of Interstate and Defense Highways'''<!--that is the full, formal name; see the talk page or first citation-->, commonly known as the '''Interstate Highway System''', or the '''Eisenhower Interstate System''', is a network of controlled-access highways that forms part of the National Highway System in the {{wipe|United States}}. The system extends throughout the contiguous United States and has routes in Hawaii, [[Earth:Alaska|Alaska]], and Puerto Rico.

In the 20th century, the United States Congress began funding roadways through the Federal Aid Road Act of 1916, and started an effort to construct a national road grid with the passage of the Federal Aid Highway Act of 1921. In 1926, the United States Numbered Highway System was established, creating the first national road numbering system for cross-country travel. The roads were state-funded and maintained, and there were few national standards for road design. United States Numbered Highways ranged from two-lane country roads to multi-lane freeways. After Dwight D. Eisenhower became president in 1953, his administration developed a proposal for an interstate highway system, eventually resulting in the enactment of the Federal-Aid Highway Act of 1956.

Unlike the earlier United States Numbered Highway System, the Interstates were designed to be an all-freeway system, with nationally unified standards for construction and signage. While some older freeways were adopted into the system, most of the routes were completely new construction, greatly expanding the freeway network in the United States. Particularly in densely populated urban areas, these new freeways were often controversial as their building necessitated the destruction of many older, well-established neighborhoods; as a result of the many freeway revolts during the 1960s and 1970s, several planned Interstates were abandoned or re-routed to avoid urban cores.

Construction of the original Interstate Highway System was proclaimed complete in 1992, despite deviations from the original 1956 plan and several stretches that did not fully conform with federal standards. The construction of the Interstate Highway System cost approximately $114&nbsp;billion (equivalent to ${{formatprice|{{inflation|US-GDP|425000000000|2006}}}} in {{inflation-year|US-GDP}}). The system has continued to expand and grow as additional federal funding has provided for new routes to be added, and many future Interstate Highways are currently either being planned or under construction.

Though much of their construction was funded by the federal government, Interstate Highways are owned by the state in which they were built. With few exceptions, all Interstates must meet specific standards, such as having controlled access, physical barriers or [[Engineering:Median strip|median strip]]s between lanes of oncoming traffic, breakdown lanes, avoiding [[Engineering:Intersection (road)|at-grade intersections]], no traffic lights and complying with federal traffic sign specifications. Interstate Highways use a numbering scheme in which primary Interstates are assigned one- or two-digit numbers, and shorter routes which branch off of longer ones are assigned three-digit numbers where the last two digits match the parent route. The Interstate Highway System is partially financed through the Highway Trust Fund, which itself is funded by a federal fuel tax. Though federal legislation initially banned the collection of tolls, some Interstate routes are [[Engineering:Toll road|toll road]]s, either because they were grandfathered into the system or because subsequent legislation has allowed for tolling of Interstates in some cases.

{{As of|2020}}, about one-quarter of all vehicle miles driven in the country used the Interstate Highway System,<ref>{{cite book |type = Report |author = ((Office of Highway Policy Information)) |publisher = Federal Highway Administration |url = https://www.fhwa.dot.gov/policyinformation/statistics/2020/vm1.cfm |title = Table VM-1: Annual Vehicle Distance Traveled in Miles and Related Data, 2020, by Highway Category and Vehicle Type |date = December 15, 2021 |access-date = May 13, 2022 }}</ref> which had a total length of {{convert|48,756|mi|km}}.<ref name="hm20">{{cite book |type = Report |author = ((Office of Highway Policy Information)) |publisher = Federal Highway Administration |url = https://www.fhwa.dot.gov/policyinformation/statistics/2020/hm20.cfm |title = Table HM-20: Public Road Length, 2020, Miles By Functional System |date = October 26, 2021 |access-date = May 13, 2022 }}</ref> In 2020 and 2021, the number of fatalities on the Interstate Highway System amounted to more than 5,000 people annually.<ref>{{cite report |author=National Center for Statistics and Analysis|date=May 2022|title=Early Estimates of Motor Vehicle Traffic Fatalities and Fatality Rate by Sub-Categories in 2021| url=https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/813298| publisher=National Highway Traffic Safety Administration|docket=DOT HS 813 298|access-date=2023-11-11}}</ref>

==History==

===Planning===
{{listen |filename=Cadillacsquareexcerpt.ogg |title=Remarks in Cadillac Square, Detroit |description=President Eisenhower delivered remarks about the need for a new highway program at Cadillac Square in Detroit on October 29, 1954<br />[http://www.fhwa.dot.gov/interstate/audiotext.cfm#cadillac Text of speech excerpt]}}
[[File:Project for the Development of National Highways of the United States.png|thumb|The Pershing Map]]
[[File:FDR Proposed Highways.jpg|thumb|FDR's hand-drawn map from 1938]]

The United States government's efforts to construct a national network of highways began on an ''ad hoc'' basis with the passage of the Federal Aid Road Act of 1916, which provided $75&nbsp;million over a five-year period for matching funds to the states for the construction and improvement of highways.<ref>{{cite book |first = Carlos Arnaldo |last = Schwantes |title = Going Places: Transportation Redefines the Twentieth-Century West |location = Bloomington |publisher = Indiana University Press |year = 2003 |isbn = 9780253342027 |page = 142 }}</ref> The nation's revenue needs associated with [[Social:World War I|World War I]] prevented any significant implementation of this policy, which expired in 1921.

In December 1918, E. J. Mehren, a civil engineer and the editor of ''Engineering News-Record'', presented his "A Suggested National Highway Policy and Plan"<ref name="mehren">{{cite magazine |first = E.J. |last = Mehren |title = A Suggested National Highway Policy and Plan |magazine = Engineering News-Record |date = December 19, 1918 |volume = 81 |issue = 25 |pages = 1112–1117 |url = https://books.google.com/books?id=d7tCAQAAMAAJ&q=%22A+suggested+national+highway+policy+and+plan%22+Dec.+19+1918&pg=PA1109 |issn = 0891-9526 |access-date = August 17, 2015 |via = [[Google Books]] }}</ref> during a gathering of the State Highway Officials and Highway Industries Association at the Congress Hotel in Chicago.<ref>{{cite web |first = Richard |last = Weingroff |date = October 15, 2013 |publisher = Federal Highway Administration |title = 'Clearly Vicious as a Matter of Policy': The Fight Against Federal-Aid |url = http://www.fhwa.dot.gov/infrastructure/hwyhist04a.cfm |access-date = August 17, 2015 |archive-url = https://web.archive.org/web/20150924032716/http://www.fhwa.dot.gov/infrastructure/hwyhist04a.cfm |archive-date = September 24, 2015 |url-status = live }}</ref> In the plan, Mehren proposed a {{convert|50,000|mi|km|adj=on}} system, consisting of five east–west routes and 10 north–south routes. The system would include two percent of all roads and would pass through every state at a cost of {{convert|25,000|$/mi|$/km}}, providing commercial as well as military transport benefits.<ref name= "mehren" />

In 1919, the US Army sent an expedition across the US to determine the difficulties that military vehicles would have on a cross-country trip. Leaving from the Ellipse near the White House on July 7, the Motor Transport Corps convoy needed 62 days to drive {{convert|3,200|mi|km}} on the Lincoln Highway to the Presidio army base on San Francisco Bay. The convoy suffered many setbacks and problems on the route, such as poor-quality bridges, broken crankshafts, and engines clogged with desert sand.<ref name="Watson article on Motor Transport convoy">{{cite magazine |last1 = Watson |first1 = Bruce |title = Ike's Excellent Adventure |magazine = American Heritage |volume = 65 |issue = 4 |date = July–August 2020 |url = https://www.americanheritage.com/ikes-excellent-adventure |access-date = July 9, 2020 |archive-date = July 9, 2020 |archive-url = https://web.archive.org/web/20200709110417/https://www.americanheritage.com/ikes-excellent-adventure |url-status = live }}</ref>

Dwight Eisenhower, then a 28-year-old brevet lieutenant colonel,<ref>{{cite book |last = Ambrose |first = Stephen |year = 1983 |title = Eisenhower: Soldier, General of the Army, President-Elect (1893–1952) |volume = 1 |location = New York |publisher = Simon & Schuster }}{{page needed|date=December 2021}}</ref> accompanied the trip "through darkest America with truck and tank," as he later described it. Some roads in the West were a "succession of dust, ruts, pits, and holes."<ref name="Watson article on Motor Transport convoy" />

As the landmark 1916 law expired, new legislation was passed—the Federal Aid Highway Act of 1921 (Phipps Act). This new road construction initiative once again provided for federal matching funds for road construction and improvement, $75&nbsp;million allocated annually.<ref name="schwantes152">{{harvp|ps=.|Schwantes|2003|p=152}}</ref> Moreover, this new legislation for the first time sought to target these funds to the construction of a national road grid of interconnected "primary highways", setting up cooperation among the various state highway planning boards.<ref name="schwantes152" />

The Bureau of Public Roads asked the Army to provide a list of roads that it considered necessary for national defense.<ref>{{cite book |last = McNichol |first = Dan |year = 2006a |title = The Roads That Built America: The Incredible Story of the U.S. Interstate System |location = New York |publisher = Sterling |isbn = 978-1-4027-3468-7 |page = 87 }}</ref> In 1922, General John J. Pershing, former head of the American Expeditionary Force in Europe during the war, complied by submitting a detailed network of {{convert|20,000|mi|km}} of interconnected primary highways—the so-called Pershing Map.<ref>{{harvp|ps=.|Schwantes|2003|p=153}}</ref>

A boom in road construction followed throughout the decade of the 1920s, with such projects as the New York parkway system constructed as part of a new national highway system. As automobile traffic increased, planners saw a need for such an interconnected national system to supplement the existing, largely non-freeway, United States Numbered Highways system. By the late 1930s, planning 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 United States marked with eight superhighway corridors for study.<ref>{{harvp|ps=.|McNichol|2006a|p=78}}</ref> In 1939, Bureau of Public Roads Division of Information chief Herbert S. Fairbank wrote a report called ''Toll Roads and Free Roads'', "the first formal description of what became the Interstate Highway System" and, in 1944, the similarly themed ''Interregional Highways''.<ref>{{cite magazine |last = Weingroff |first = Richard F. |title = The Federal-State Partnership at Work: The Concept Man |magazine = Public Roads |volume = 60 |issue = 1 |date = Summer 1996 |url = http://www.tfhrc.gov/pubrds/summer96/p96su7b.htm#9 |access-date = March 16, 2012 |issn = 0033-3735 |archive-url = https://web.archive.org/web/20100528132734/http://www.tfhrc.gov/pubrds/summer96/p96su7b.htm#9 |archive-date = May 28, 2010 |url-status = dead }}</ref>

===Federal Aid Highway Act of 1956===

The Interstate Highway System gained a champion in President Dwight D. Eisenhower, who was influenced by his experiences as a young Army officer crossing the country in the 1919 Motor Transport Corps convoy that drove in part on the Lincoln Highway, the first road across America. He recalled that, "The old convoy had started me thinking about good two-lane highways... the wisdom of broader ribbons across our land."<ref name="Watson article on Motor Transport convoy" /> Eisenhower also gained an appreciation of the Reichsautobahn system, the first "national" implementation of modern Germany's Autobahn network, as a necessary component of a national defense system while he was serving as Supreme Commander of Allied Forces in Europe during World War II.<ref>{{cite magazine |title = On the Road |first = Henry |last = Petroski |magazine = American Scientist |volume = 94 |issue = 5 |year = 2006 |pages = 396–369 |issn = 0003-0996 |doi = 10.1511/2006.61.396 }}</ref> In 1954, Eisenhower appointed General Lucius D. Clay to head a committee charged with proposing an interstate highway system plan.<ref>{{cite book |last = Smith |first = Jean Edward |title = Eisenhower in War and Peace |publisher = Random House |isbn = 978-1400066933 |date = 2012 |page = 652 }}</ref> Summing up motivations for the construction of such a system, Clay stated,
{{Blockquote|It was evident we needed better highways. We needed them for safety, to accommodate more automobiles. We needed them for defense purposes, if that should ever be necessary. And we needed them for the economy. Not just as a public works measure, but for future growth.<ref>{{harvp|Smith|2012|pp=652–653|ps=.}}</ref>}}

Clay's committee proposed a 10-year, $100&nbsp;billion program, which would build {{convert|40,000|mi|km}} of divided highways linking all American cities with a population of greater than 50,000. Eisenhower initially preferred a system consisting of [[Engineering:Toll road|toll road]]s, but Clay convinced Eisenhower that toll roads were not feasible outside of the highly populated coastal regions. In February 1955, Eisenhower forwarded Clay's proposal to Congress. The bill quickly won approval in the Senate, but House Democrats objected to the use of public [[Finance:Bond|bonds]] as the means to finance construction. Eisenhower and the House Democrats agreed to instead finance the system through the Highway Trust Fund, which itself would be funded by a [[Chemistry:Gasoline|gasoline]] tax.<ref>{{harvp|Smith|2012|pp=651–654|ps=.}}</ref> In June 1956, Eisenhower signed the Federal Aid Highway Act of 1956 into law. Under the act, the federal government would pay for 90 percent of the cost of construction of Interstate Highways. Each Interstate Highway was required to be a freeway with at least four lanes and no at-grade crossings.<ref>{{cite web |title = The Interstate Highway System |url = https://www.history.com/topics/us-states/interstate-highway-system |website = History |publisher = A&E Television Networks |access-date = May 10, 2019 |archive-url = https://web.archive.org/web/20190510175042/https://www.history.com/topics/us-states/interstate-highway-system |archive-date = May 10, 2019 |url-status = live }}</ref>

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 Highway System.<ref>{{cite web |url = http://etext.virginia.edu/journals/EH/EH38/Norton.html |title = Fighting Traffic: U.S. Transportation Policy and Urban Congestion, 1955–1970 |last = Norton |first = Peter |year = 1996 |access-date = January 17, 2008 |work = Essays in History |publisher = Corcoran Department of History at the University of Virginia |url-status = dead |archive-url = https://web.archive.org/web/20080215220316/http://etext.virginia.edu/journals/EH/EH38/Norton.html |archive-date = February 15, 2008 }}</ref> Assisting in the planning was Charles Erwin Wilson, who was still head of [[Company:General Motors|General Motors]] when President Eisenhower selected him as Secretary of Defense in January 1953.

===Construction===
[[File:National Highway Program - National System of Interstate Highways - Rural Status of Improvement, 1965.png|thumb|1955 map: The planned status of US Highways in 1965, as a result of the developing Interstate Highway System]]
[[File:STEEL RODS, MADE FROM SHREDDED AUTOS, ARE BEING USED FOR REINFORCEMENT IN THIS SECTION OF I-55, NORTH OF DURANT. IT... - NARA - 546265.jpg|thumb|upright|I‑55 under construction in Mississippi, photo from May 1972]]

Some sections of highways that became part of the Interstate Highway System actually began construction earlier.

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 upgrading a section of US Route 66 to what is now designated Interstate 44.<ref name="weingroff">{{cite magazine |url = http://www.fhwa.dot.gov/publications/publicroads/96summer/p96su18.cfm |title = Three States Claim First Interstate Highway |last = Weingroff |first = Richard F. |date = Summer 1996 |volume = 60 |issue = 1 |magazine = Public Roads |access-date = February 16, 2008 |issn = 0033-3735 |archive-url = https://web.archive.org/web/20101011155643/http://www.fhwa.dot.gov/publications/publicroads/96summer/p96su18.cfm |archive-date = October 11, 2010 |url-status = live }}</ref> On August 13, 1956, work began on US 40 (now I-70) in St. Charles County.<ref name=Sherrill>{{cite news |url = https://www.journalnow.com/news/local/facts-and-history-of-north-carolina-interstates/article_cf1a0399-801f-57f5-a0a0-ab4ec56851d5.html |title = Facts and History of North Carolina Interstates |last = Sherrill |first = Cassandra |work = Winston-Salem Journal |date = September 28, 2019 |access-date = September 29, 2019 |archive-date = September 29, 2019 |archive-url = https://web.archive.org/web/20190929193542/https://www.journalnow.com/news/local/facts-and-history-of-north-carolina-interstates/article_cf1a0399-801f-57f5-a0a0-ab4ec56851d5.html |url-status = live }}</ref><ref name="weingroff"/>

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.<ref name="weingroff" />

The Pennsylvania Turnpike could also be considered one of the first Interstate Highways, and is nicknamed "Grandfather of the Interstate System".<ref name=Sherrill/> On October 1, 1940, {{convert|162|mi|km|0}} 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, a reference to [[Engineering:Toll road|turnpike]]s.<ref name="weingroff" />

Milestones in the construction of the Interstate Highway System include:
* October 17, 1974: Nebraska becomes the first state to complete all of its mainline Interstate Highways with the dedication of its final piece of I-80.<ref>{{cite web |url = http://www.nebraskatransportation.org/i-80-anniv/index.htm |title = I-80 50th Anniversary Page |author = Nebraska Department of Roads |date = n.d. |publisher = Nebraska Department of Roads |access-date = August 23, 2009 |archive-url = https://web.archive.org/web/20131221052401/http://www.transportation.nebraska.gov/i-80-anniv/index.htm |archive-date = December 21, 2013 |url-status = live |}}</ref>
* October 12, 1979: The final section of the Canada to Mexico freeway Interstate 5 is dedicated near Stockton, California. Representatives of the two neighboring nations attended the dedication to commemorate the first contiguous freeway connecting the North American countries.<ref>{{cite web |url = http://www.dot.ca.gov/interstate/timeline.htm |author = California Department of Transportation |date = n.d. |publisher = California Department of Transportation |title = Timeline of Notable Events of the Interstate Highway System in California |access-date = March 2, 2014 |archive-url = https://web.archive.org/web/20140306100816/http://www.dot.ca.gov/interstate/timeline.htm |archive-date = March 6, 2014 |url-status = dead |}}</ref>
* August 22, 1986: The final section of the coast-to-coast I-80 ({{wipe|San Francisco, California}}, to Teaneck, New Jersey) is dedicated on the western edge of Salt Lake City, Utah, making I-80 the world's first contiguous freeway to span from the Atlantic to Pacific Ocean and, at the time, the longest contiguous freeway in the world. The section spanned from Redwood Road to just west of the Salt Lake City International Airport. At the dedication it was noted that coincidentally this was only {{convert|50|mi|km}} from Promontory Summit, where a similar feat was accomplished nearly 120&nbsp;years prior, the driving of the golden spike of the United States' First transcontinental railroad.<ref name="highway history">{{cite magazine |url = http://www.fhwa.dot.gov/infrastructure/30thannv.cfm |title = America Celebrates 30th Anniversary of the Interstate System |magazine = US Highways |date = Fall 1986 |access-date = March 10, 2012 |archive-url = https://web.archive.org/web/20111024114212/https://www.fhwa.dot.gov/infrastructure/30thannv.cfm |archive-date = October 24, 2011 |url-status = dead }}</ref><ref name="nytimes">{{cite news |url = https://www.nytimes.com/1986/08/25/us/around-the-nation-transcontinental-road-completed-in-utah.html |title = Around the Nation: Transcontinental Road Completed in Utah |work = {{wipe|The New York Times}} |date = August 25, 1986 |access-date = February 9, 2017 |archive-url = https://web.archive.org/web/20170316115134/http://www.nytimes.com/1986/08/25/us/around-the-nation-transcontinental-road-completed-in-utah.html |archive-date = March 16, 2017 |url-status = live }}</ref><ref name="utahmap">{{cite map |author = Utah Transportation Commission |year = 1983 |title = Official Highway Map |scale = Scale not given |location = Salt Lake City |publisher = Utah Department of Transportation |inset = Salt Lake City }}</ref>
* August 10, 1990: The final section of coast-to-coast I-10 (Santa Monica, California, to Jacksonville, Florida) is dedicated, the Papago Freeway Tunnel under downtown Phoenix, Arizona. Completion of this section was delayed due to a freeway revolt that forced the cancellation of an originally planned elevated routing.<ref name="pr">{{cite magazine |url = http://www.fhwa.dot.gov/publications/publicroads/06jan/01.cfm |title = The Year of the Interstate |last = Weingroff |first = Richard F. |magazine = Public Roads |date = January 2006 |volume = 69 |issue = 4 |issn = 0033-3735 |access-date = March 10, 2012 |archive-url = https://web.archive.org/web/20120104024139/http://www.fhwa.dot.gov/publications/publicroads/06jan/01.cfm |archive-date = January 4, 2012 |url-status = live }}</ref>
* September 12, 1991: I-90 becomes the final coast-to-coast Interstate Highway (Seattle, Washington to Boston, Massachusetts) to be completed with the dedication of an elevated viaduct bypassing Wallace, Idaho, which opened a week earlier.<ref>{{cite news |last=Devlin |first=Sherry |date=September 8, 1991 |title=No Stopping Now |page=E1 |work=The Missoulian |url=https://www.newspapers.com/clip/90334418/no-stopping-now/ |via=Newspapers.com |accessdate=September 12, 2023}}</ref><ref>{{cite news |last=Free |first=Cathy |date=September 15, 1991 |title=Engineer pleased with his Wallace freeway 'work of art' |page=B3 |work=The Spokesman-Review |url=https://www.newspapers.com/clip/89735440/engineer-pleased-with-his-wallace/ |via=Newspapers.com |accessdate=September 12, 2023}}</ref> This section was delayed after residents forced the cancellation of the originally planned at-grade alignment that would have demolished much of downtown Wallace. The residents accomplished this feat by arranging for most of the downtown area to be declared a historic district and listed on the National Register of Historic Places; this succeeded in blocking the path of the original alignment. Two days after the dedication residents held a mock funeral celebrating the removal of the last stoplight on a transcontinental Interstate Highway.<ref name="pr" /><ref>{{cite web |url = http://www.itd.idaho.gov/50.years/I-50_I-90.html |author = Idaho Transportation Department |publisher = Idaho Transportation Department |title = Celebrating 50 years of Idaho's Interstates |date = May 31, 2006 |access-date = March 10, 2012 |url-status = dead |archive-url = https://web.archive.org/web/20120224045451/http://itd.idaho.gov/50.Years/I-50_I-90.html |archive-date = February 24, 2012 |}}</ref>
* October 14, 1992: The original Interstate Highway System is proclaimed to be complete with the opening of I-70 through Glenwood Canyon in Colorado. This section is considered an engineering marvel with a {{convert|12|mi|km|adj=on}} span featuring 40&nbsp;bridges and numerous tunnels and is one of the most expensive rural highways per mile built in the United States.<ref name="funfacts">{{cite web |author = Colorado Department of Transportation |date = n.d. |url = http://www.dot.state.co.us/50anniversary/funfacts.cfm |title = CDOT Fun Facts |access-date = February 15, 2008 |publisher = Colorado Department of Transportation |url-status = dead |archive-url = https://web.archive.org/web/20080116125059/http://www.dot.state.co.us/50anniversary/funfacts.cfm |archive-date = January 16, 2008 |author-link = Colorado Department of Transportation }}</ref><ref name="12yearslater">{{cite web |url = http://www.tfhrc.gov/pubrds/04mar/04.htm |title = Glenwood Canyon 12 Years Later |first1 = Karen |last1 = Stufflebeam Row |first2 = Eva |last2 = LaDow |first3 = Steve |last3 = Moler |name-list-style = amp |date = March 2004 |publisher = Federal Highway Administration |access-date = May 11, 2008 |archive-url = https://web.archive.org/web/20130117094404/http://www.fhwa.dot.gov/publications/publicroads/04mar/ |archive-date = January 17, 2013 |url-status = dead }}</ref>

The initial cost estimate for the system was $25&nbsp;billion over 12&nbsp;years; it ended up costing $114&nbsp;billion (equivalent to $425&nbsp;billion in 2006<ref name="usatoday062206">{{cite news |work = [[Software:USA Today|USA Today]] |last = Neuharth |first = Al |date = June 22, 2006 |title = Traveling Interstates is our Sixth Freedom |url = https://www.usatoday.com/news/opinion/columnist/neuharth/2006-06-22-interstates_x.htm |access-date = May 9, 2012 |archive-url = https://web.archive.org/web/20120819092803/http://www.usatoday.com/news/opinion/columnist/neuharth/2006-06-22-interstates_x.htm |archive-date = August 19, 2012 |url-status = live }}</ref> or ${{formatprice|{{inflation|US-GDP|425000000000|2006}}}} in {{inflation-year|US-GDP}}{{inflation-fn|US-GDP}}) and took 35&nbsp;years.<ref name="mndot50">{{cite web |author = Minnesota Department of Transportation |url = http://www.dot.state.mn.us/interstate50/50facts.html |title = Mn/DOT Celebrates Interstate Highway System's 50th Anniversary |year = 2006 |access-date = January 17, 2008 |publisher = Minnesota Department of Transportation |archive-url = https://web.archive.org/web/20071204072603/http://www.dot.state.mn.us/interstate50/50facts.html <!-- Bot retrieved archive --> |archive-date = December 4, 2007 |}}</ref>

===1992–present===

====Discontinuities====
[[File:Eisenhower Interstate System sign.jpg|thumb|left|Commemorative sign introduced in 1993. The system was established during Dwight D. Eisenhower's presidency, and the five stars commemorate his rank as General of the Army during World War II.]]

The system was proclaimed complete in 1992, but two of the original Interstates—I-95 and I-70—were not continuous: both of these discontinuities were due to local opposition, which blocked efforts to build the necessary connections to fully complete the system. I-95 was made a continuous freeway in 2018,<ref name="sofield">{{cite news |first = Tom |last = Sofield |date = September 22, 2018 |title = Decades in the Making, I-95, Turnpike Connector Opens to Motorists |url = http://levittownnow.com/2018/09/22/decades-in-the-making-i-95-turnpike-connector-opens-to-motorists/ |work = Levittown Now |access-date = September 22, 2018 |archive-date = April 6, 2020 |archive-url = https://web.archive.org/web/20200406195324/http://levittownnow.com/2018/09/22/decades-in-the-making-i-95-turnpike-connector-opens-to-motorists/ |url-status = live}}</ref> and thus I-70 remains the only original Interstate with a discontinuity.

I-95 was discontinuous in New Jersey because of the cancellation of the Somerset Freeway. This situation was remedied when the construction of the Pennsylvania Turnpike/Interstate 95 Interchange Project started in 2010<ref name="PA">{{cite web |author = Pennsylvania Turnpike Commission |date = n.d. |url = http://www.paturnpikei95.com/pdf/DACMeeting050914.pdf |publisher = Pennsylvania Turnpike Commission |work = I-95/I-276 Interchange Project Meeting Design Management Summary |title = Draft: Design Advisory Committee Meeting No. 2 |access-date = May 11, 2008 |archive-url = https://web.archive.org/web/20131002051518/http://www.paturnpikei95.com/pdf/DACMeeting050914.pdf |archive-date = October 2, 2013 |url-status = dead |}}</ref> and partially opened on September 22, 2018, which was already enough to fill the gap.<ref name="sofield" />

However, I-70 remains discontinuous in Pennsylvania, because of the lack of a direct interchange with the Pennsylvania Turnpike at the eastern end of the concurrency near Breezewood. Traveling in either direction, I-70 traffic must exit the freeway and use a short stretch of US&nbsp;30 (which includes a number of roadside services) to rejoin I-70. The interchange was not originally built because of a legacy federal funding rule, since relaxed, which restricted the use of federal funds to improve roads financed with tolls.<ref name="fhwa.dot.gov">{{cite web |author = Federal Highway Administration |url = http://www.fhwa.dot.gov/infrastructure/tollroad.cfm |title = Why Does The Interstate System Include Toll Facilities? |publisher = Federal Highway Administration |date = n.d. |access-date = July 15, 2009 |archive-url = https://web.archive.org/web/20130518082124/http://www.fhwa.dot.gov/infrastructure/tollroad.cfm |archive-date = May 18, 2013 |url-status = live }}</ref> Solutions have been proposed to eliminate the discontinuity, but they have been blocked by local opposition, fearing a loss of business.<ref>{{cite news |title = Dawida seeks to merge I-70, turnpike at Breezewood |first = Gary |last = Tuna |work = Pittsburgh Post-Gazette |date = July 27, 1989 |via = Google News |url = https://news.google.com/newspapers?id=O7RRAAAAIBAJ&pg=4854,6978383&dq=robert-jubelirer+breezewood&hl=en |access-date = November 19, 2015 |archive-url = https://web.archive.org/web/20150930132552/https://news.google.com/newspapers?id=O7RRAAAAIBAJ&sjid=Wm4DAAAAIBAJ&pg=4854,6978383&dq=robert-jubelirer+breezewood&hl=en |archive-date = September 30, 2015 |url-status = live }}</ref>

====Expansions and removals====
The Interstate Highway System has been expanded numerous times. The expansions have both created new designations and extended existing designations. For example, I-49, added to the system in the 1980s as a freeway in Louisiana, was designated as an expansion corridor, and FHWA approved the expanded route north from Lafayette, Louisiana, to Kansas City, Missouri. The freeway exists today as separate completed segments, with segments under construction or in the planning phase between them.<ref>{{cite web |url = http://www.modot.org/southwest_archive/I-49MapsandInformation.htm |author = Missouri Department of Transportation |date = n.d. |publisher = Missouri Department of Transportation |work = Interstate I-49 Expansion Corridor in Southwest District of Missouri |title = Converting US Route 71 to I-49 |url-status = dead |archive-url = https://web.archive.org/web/20130117094333/http://www.modot.org/southwest_archive/I-49MapsandInformation.htm |archive-date = January 17, 2013 |}}</ref>

In 1966, the FHWA designated the entire Interstate Highway System as part of the larger Pan-American Highway System,<ref>{{cite book |author = New Mexico State Highway and Transportation Department |url = http://www.nmshtd.state.nm.us/upload/contents/445/Memorial.pdf |title = State of New Mexico Memorial Designations and Dedications of Highways, Structures and Buildings |year = 2007 |location = Santa Fe |publisher = New Mexico State Highway and Transportation Department |page = 14 |url-status = dead |archive-url = https://web.archive.org/web/20110716081405/http://www.nmshtd.state.nm.us/upload/contents/445/Memorial.pdf |archive-date = July 16, 2011 |author-link = New Mexico State Highway and Transportation Department }}</ref> and at least two proposed Interstate expansions were initiated to help trade with Canada and Mexico spurred by the [[Finance:North American Free Trade Agreement|North American Free Trade Agreement]] (NAFTA). Long-term plans for I-69, which currently exists in several separate completed segments (the largest of which are in {{wipe|Indiana}} and {{wipe|Texas}}), is to have the highway route extend from Tamaulipas, Mexico to Ontario, Canada. The planned I-11 will then bridge the Interstate gap between Phoenix, Arizona and Las Vegas, Nevada, and thus form part of the CANAMEX Corridor (along with I-19, and portions of I-10 and I-15) between [[Place:Sonora|Sonora]], Mexico and Alberta, Canada.

===Local opposition and cancellations===
Political opposition from residents canceled many freeway projects around the United States, including:
* I-40 in Memphis, Tennessee was rerouted and part of the original I-40 is still in use as the eastern half of Sam Cooper Boulevard.<ref>{{harvp|ps=.|McNichol|2006a|pp=159–160}}</ref>
* I-66 in the District of Columbia was abandoned in 1977.
* I-69 was to continue past its terminus at Interstate 465 to intersect with Interstate 70 and Interstate 65 at the north split, northeast of downtown [[Place:Indianapolis|Indianapolis]]. Though local opposition led to the cancellation of this project in 1981, bridges and ramps for the connection into the "north split" remain visible.
* I-70 in Baltimore was supposed to run from the Baltimore Beltway (Interstate 695), which surrounds the city to terminate at I-95, the East Coast thoroughfare that runs through Maryland and Baltimore on a diagonal course, northeast to southwest; the connection was cancelled on the mid-1970s due to its routing through Gwynns Falls-Leakin Park, a wilderness urban park reserve following the Gwynns Falls stream through West Baltimore. This included the cancellation of I-170, partially built and in use as US&nbsp;40, and nicknamed the Highway to Nowhere.
* I-78 in New York City was canceled along with portions of I-278, I-478, and I-878. I-878 was supposed to be part of I-78, and I-478 and I-278 were to be spur routes.
* I-80 in San Francisco was originally planned to travel past the city's Civic Center along the Panhandle Freeway into Golden Gate Park and terminate at the original alignment of I-280/SR 1. The city canceled this and several other freeways in 1958. Similarly, more than 20 years later, Sacramento canceled plans to upgrade I-80 to Interstate Standards and rerouted the freeway on what was then I-880 that traveled north of Downtown Sacramento.
* I-83, southern extension of the Jones Falls Expressway (southern I-83) in Baltimore was supposed to run along the waterfront of the Patapsco River / Baltimore Harbor to connect to I-95, bisecting historic neighborhoods of Fells Point and Canton, but the connection was never built.
* I-84 in Connecticut was once planned to fork east of Hartford, into an I-86 to Sturbridge, Massachusetts, and I-84 to Providence, R.I. The plan was cancelled, primarily because of anticipated impact on a major Rhode Island reservoir. The I-84 designation was restored to the highway to Sturbridge, and other numbering was used for completed Eastern sections of what had been planned as part of I-84.
* I-95 through the District of Columbia into Maryland was abandoned in 1977. Instead it was rerouted to I-495 (Capital Beltway). The completed section is now I-395.
* I-95 was originally planned to run up the Southwest Expressway and meet I-93, where the two highways would travel along the Central Artery through downtown Boston, but was rerouted onto the Route 128 beltway due to widespread opposition. This revolt also included the cancellation of the Inner Belt, connecting I-93 to I-90 and a cancelled section of the Northwest Expressway which would have carried US&nbsp;3 inside the Route 128 beltway, meeting with Route 2 in Cambridge.

==Standards==
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===
[[File:2019-07-15 11 10 50 View south along Interstate 95 from the overpass for Maryland State Route 175 (Waterloo Road-Rouse Parkway) in Columbia, Howard County, Maryland.jpg|alt=|thumb|I-95 in Columbia, Maryland, built to modern standards.]]
[[File:Interstate_5_in_the_Central_Valley.jpg|thumb|right|A rural stretch of I-5 in California; two lanes in each direction are separated by a large grassy median and cross-traffic is limited to grade separations such as this overpass.]]
Being freeways, Interstate Highways usually have the highest speed limits in a given area. Speed limits are determined by individual states. From 1975 to 1986, the maximum speed limit on any highway in the United States was {{Convert|55|mi/h|km/h|round=5}}, in accordance with federal law.<ref name="nytimes3a">{{cite news |title = Nixon Approves Limit of 55 M.P.H. |url = http://select.nytimes.com/gst/abstract.html?res=F40610FD3F58137B93C1A9178AD85F408785F9&scp=1&sq=nixon%20approves%20limit%20of%2055&st=cse |access-date = July 27, 2008 |work = The New York Times |pages = 1, 24 |date = January 3, 1974 |url-access = subscription |archive-url = https://web.archive.org/web/20110605095932/http://select.nytimes.com/gst/abstract.html?res=F40610FD3F58137B93C1A9178AD85F408785F9&scp=1&sq=nixon%20approves%20limit%20of%2055&st=cse |archive-date= June 5, 2011 |url-status= dead}}</ref>

Typically, lower limits are established in Northeastern and coastal states, while higher speed limits are established in inland states west of the Mississippi River.<ref name="mit">{{cite web |url = https://www.mit.edu/~jfc/laws.html |title = State traffic and speed laws |date = October 11, 2007 |access-date = January 10, 2008 |publisher = Massachusetts Institute of Technology |last = Carr |first = John |archive-url = https://web.archive.org/web/20130807221607/http://www.mit.edu/~jfc/laws.html |archive-date = August 7, 2013 |url-status = live }}</ref> For example, the maximum speed limit is {{convert|75|mph|km/h|round=5|abbr=on}} in northern Maine, varies between {{convert|50|and|70|mph|km/h|round=5|abbr=on}}<ref>{{cite news |first = Paul |last = Koenig |date = May 27, 2014 |title = Speed Limit on Much of I-295 Rises to 70&nbsp;MPH |url = http://www.pressherald.com/2014/05/27/speed_limt_on_much_of_maine_turnpike__i-295_ro_rise_to_70_mph/ |work = Portland Press Herald |access-date = July 22, 2014 |archive-url = https://web.archive.org/web/20140727171449/http://www.pressherald.com/2014/05/27/speed_limt_on_much_of_maine_turnpike__i-295_ro_rise_to_70_mph/ |archive-date = July 27, 2014 |url-status = live }}</ref> from southern Maine to New Jersey, and is {{convert|50|mph|km/h|round=5|abbr=on}} in New York City and the District of Columbia.<ref name="mit" /> Currently, rural speed limits elsewhere generally range from {{convert|65|to|80|mph|km/h|round=5}}. Several portions of various highways such as I-10 and I-20 in rural western Texas, I-80 in Nevada between Fernley and Winnemucca (except around Lovelock) and portions of I-15, I-70, I-80, and I-84 in Utah have a speed limit of {{convert|80|mph|km/h|round=5|abbr=on}}. Other Interstates in Idaho, Montana, Oklahoma, South Dakota and Wyoming also have the same high speed limits.

In some areas, speed limits on Interstates can be significantly lower in areas where they traverse significantly hazardous areas. The maximum speed limit on I-90 is {{convert|50|mph|km/h|round=5|abbr=on}} in downtown Cleveland because of two sharp curves with a suggested limit of {{convert|35|mph|km/h|round=5|abbr=on}} in a heavily congested area; I-70 through Wheeling, West Virginia, has a maximum speed limit of {{convert|45|mph|km/h|round=5|abbr=on}} through the Wheeling Tunnel and most of downtown Wheeling; and I-68 has a maximum speed limit of {{convert|40|mph|km/h|round=5|abbr=on}} through Cumberland, Maryland, because of multiple hazards including sharp curves and narrow lanes through the city. In some locations, low speed limits are the result of lawsuits and resident demands; after holding up the completion of I-35E in St. Paul, Minnesota, for nearly 30&nbsp;years in the courts, residents along the stretch of the freeway from the southern city limit to downtown successfully lobbied for a {{convert|45|mph|km/h|round=5|abbr=on}} speed limit in addition to a prohibition on any vehicle weighing more than {{convert|9,000|lbs|kg}} gross vehicle weight. I-93 in Franconia Notch State Park in northern New Hampshire has a speed limit of {{convert|45|mph|km/h|round=5|abbr=on}} because it is a parkway that consists of only one lane per side of the highway. On the other hand, Interstates 15, 80, 84, and 215 in Utah have speed limits as high as {{convert|70|mph|km/h|round=5|abbr=on}} within the Wasatch Front, Cedar City, and [[Place:St. George, Utah|St. George]] areas, and I-25 in New Mexico within the Santa Fe and Las Vegas areas along with I-20 in Texas along Odessa and Midland and I-29 in North Dakota along the Grand Forks area have higher speed limits of {{convert|75|mph|km/h|round=5|abbr=on}}.

===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 US Department of Defense.<ref name="slater_1996">{{cite magazine |url = http://www.fhwa.dot.gov/publications/publicroads/96spring/p96sp2.cfm |title = The National Highway System: A Commitment to America's Future |magazine = Public Roads |last = Slater |first = Rodney E. |date = Spring 1996 |volume = 59 |issue = 4 |access-date = January 10, 2008 |issn = 0033-3735 |archive-url = https://web.archive.org/web/20141216112008/http://www.fhwa.dot.gov/publications/publicroads/96spring/p96sp2.cfm |archive-date = December 16, 2014 |url-status = live }}</ref>

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 [[Social:Contraflow lane reversal|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 I-16 and I-26 were used in a contraflow configuration in anticipation of Hurricane Floyd with mixed results.<ref name="onewayout">{{cite magazine |url = http://www.asce.org/files/pdf/hurricane/Evacuation_and_Shelters/One_Way_Out~Contraflow_Freeway_Operation_for_Hurricane_Evacuation.pdf |title = "One-Way-Out": Contraflow Freeway Operation for Hurricane Evacuation |last = Wolshon |first = Brian |magazine = Natural Hazards Review |volume = 2 |issue = 3 |pages = 105–112 |date = August 2001 |access-date = January 10, 2008 |archive-url = https://web.archive.org/web/20081006200038/http://www.asce.org/files/pdf/hurricane/Evacuation_and_Shelters/One_Way_Out~Contraflow_Freeway_Operation_for_Hurricane_Evacuation.pdf |archive-date = October 6, 2008 |doi = 10.1061/(ASCE)1527-6988(2001)2:3(105) }}</ref>

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;<ref name="contraflow_ivan">{{cite web |url = http://www.floridaits.com/PDFs/TWO60-Contraflow/060330-Experiences-V2.pdf |title = Contraflow Implementation Experiences in the Southern Coastal States |publisher = Florida Department of Transportation |access-date = September 27, 2007 |archive-url = https://web.archive.org/web/20071025045623/http://www.floridaits.com/PDFs/TWO60-Contraflow/060330-Experiences-V2.pdf |archive-date = October 25, 2007 |first = Tahira |last = Faquir |date = March 30, 2006 }}</ref> 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 much more smoothly.<ref name="roadsandbridges_contraflow">{{cite web |url = https://www.roadsbridges.com/contra-productive |work = Roads & Bridges |date = December 2006b |access-date = January 10, 2008 |last = McNichol |first = Dan |title = Contra Productive |archive-url = https://web.archive.org/web/20110715191558/http://www.roadsbridges.com/popup_app/index.cfm?fuseaction=showArticle&appDirectory=rb&articleID=7519&forPrint=yes |archive-date = July 15, 2011 }}</ref>

According to [[Social:Urban legend|urban legend]], early regulations required 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. There is no evidence of this rule being included in any Interstate legislation.<ref>{{cite web |first = Barbara |last = Mikkelson |date = April 1, 2011 |url = http://www.snopes.com/autos/law/airstrip.asp |title = Interstate Highways as Airstrips |publisher = Snopes |access-date = March 15, 2017 |archive-url = https://wayback.archive-it.org/all/20051201041356/http://www.snopes.com/autos/law/airstrip.asp |archive-date = December 1, 2005 |url-status = live }}</ref><ref>{{cite magazine |url = http://www.fhwa.dot.gov/publications/publicroads/00mayjun/onemileinfive.cfm |title = One Mile in Five: Debunking the Myth |last = Weingroff |first = Richard F. |date = May–June 2000 |magazine = Public Roads |volume = 63 |issue = 6 |access-date = December 14, 2010 |issn = 0033-3735 |archive-url = https://web.archive.org/web/20101212070757/http://www.fhwa.dot.gov/publications/publicroads/00mayjun/onemileinfive.cfm |archive-date = December 12, 2010 |url-status = live }}</ref>

==Numbering system==
===Primary (one- and two-digit) Interstates===
<!--Interstate 50 re-directs to this section. If this section is renamed, please update the re-direct.-->
[[File:Interstate Highway System numbering method explanation diagram.png|alt=Odd numbers run north–south with numbers increasing from west to east, while even numbers run east–west with numbers increasing from south to north.|thumb|Odd numbers run north–south with numbers increasing from west to east, while even numbers run east–west with numbers increasing from south to north.]]
[[File:I-78-US 22 EB at mile marker 24.5.JPG|thumb|I‑78 and {{nowrap|US 22}} in Berks County, Pennsylvania (2008)]]

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.<ref name="aashto_ho2">{{cite web |url = http://cms.transportation.org/sites/route/docs/HO2_Policy_Retention_HO1.pdf |title = Establishment of a Marking System of the Routes Comprising the National System of Interstate and Defense Highways |author = American Association of State Highway and Transportation Officials |publisher = American Association of State Highway and Transportation Officials |date = January 2000 |access-date = January 23, 2008 |url-status = dead |archive-url = https://web.archive.org/web/20061101234238/http://cms.transportation.org/sites/route/docs/HO2_Policy_Retention_HO1.pdf |archive-date = November 1, 2006 |}}</ref> Within the contiguous United States, primary Interstates—also called main line Interstates or two-digit Interstates—are assigned numbers less than 100.<ref name="aashto_ho2" />

While numerous exceptions do exist, there is a general scheme for numbering Interstates. Primary Interstates are assigned one- or two-digit numbers, while shorter routes (such as spurs, loops, and short connecting roads) are assigned three-digit numbers where the last two digits match the parent route (thus, I-294 is a loop that connects at both ends to I-94, while I-787 is a short spur route attached to I-87). In the numbering scheme for the primary routes, 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 (to avoid confusion with the US Highways, which increase from east to west and north to south).<ref>{{cite news |last = Fausset |first = Richard |date = November 13, 2001 |title = Highway Numerology Muddled by Potholes in Logic |page = B2 |work = {{wipe|Los Angeles Times}} |url = https://articles.latimes.com/2001/nov/13/local/me-3653 |access-date = September 8, 2018 |archive-url = https://web.archive.org/web/20090402132246/http://articles.latimes.com/2001/nov/13/local/me-3653 |archive-date = April 2, 2009 |url-status = live }}</ref> This numbering system usually holds true even if the local direction of the route does not match the compass directions. Numbers [[Division (mathematics)|divisible]] by five are intended to be major arteries among the primary routes, carrying traffic long distances.<ref>{{harvp|ps=.|McNichol|2006a|p=172}}</ref><ref name="rambler_20050118">{{cite web |url = http://www.fhwa.dot.gov/infrastructure/i76.cfm |work = Ask the Rambler |title = Was I-76 Numbered to Honor Philadelphia for Independence Day, 1776? |last = Weingroff |first = Richard F. |publisher = Federal Highway Administration |date = January 18, 2005 |access-date = January 17, 2008 |archive-url = https://web.archive.org/web/20130703012425/https://www.fhwa.dot.gov/infrastructure/i76.cfm |archive-date = July 3, 2013 |url-status = live }}</ref> Primary north–south Interstates increase in number from I-5 between Canada and Mexico along the West Coast to I‑95 between Canada and Miami, Florida 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 are no I-50 and I-60, as routes with those numbers would likely pass through states that currently have US Highways with the same numbers, which is generally disallowed under highway administration guidelines.<ref name="aashto_ho2" /><ref name="fhwa-faq19" >{{cite web |author = Federal Highway Administration |date = n.d. |url = http://www.fhwa.dot.gov/interstate/faq.htm#question19 |title = Interstate FAQ |publisher = Federal Highway Administration |access-date = June 26, 2009 |quote = 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 US Highways with the same numbers will be retained in the states upon completion of the Interstate routes. |archive-url = https://web.archive.org/web/20130507121442/http://www.fhwa.dot.gov/interstate/faq.htm#question19 |archive-date = May 7, 2013 |url-status = live }}</ref>

Several two-digit numbers are shared between unconnected road segments at opposite ends of the country for various reasons. Some such highways are incomplete Interstates (such as I-69 and I-74) and some just happen to share route designations (such as I-76, I-84, I‑86, I-87, 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.<ref name="aashto_ho2" /> Additionally, due to Congressional requirements, three sections of I-69 in southern Texas will be divided into I-69W, I-69E, and I-69C (for Central).<ref>{{cite news |last = Essex |first = Allen |title = State Adds I-69 to Interstate System |url = http://brownsvilleherald.com/news/valley/article_cbb0e04a-c99b-11e2-8c72-001a4bcf6878.html?mode=jqm |access-date = July 17, 2013 |newspaper = The Brownsville Herald |date = May 2013 |archive-url = https://web.archive.org/web/20170227191039/http://www.brownsvilleherald.com/news/valley/article_cbb0e04a-c99b-11e2-8c72-001a4bcf6878.html?mode=jqm |archive-date = February 27, 2017 |url-status = dead }}</ref>

AASHTO policy allows dual numbering to provide continuity between major control points.<ref name="aashto_ho2" /> This is referred to as a concurrency or overlap. For example, I‑75 and I‑85 share the same roadway in Atlanta; this {{convert|7.4|mi|km|adj=on}} section, called the Downtown Connector, is labeled both I‑75 and I‑85. Concurrencies between Interstate and US Highway numbers are also allowed in accordance with AASHTO policy, as long as the length of the concurrency is reasonable.<ref name="aashto_ho2" /> In rare instances, two highway designations 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===
[[File:FHWA Auxiliary Route Numbering Diagram.svg|thumb|upright=2.5|Examples of the auxiliary Interstate Highway numbering system. An odd hundreds digit means the route connects at only one end to the rest of the interstate system, known as a "spur route" (see I-310 and I-510 in image). An even hundreds digit means the route connects at both ends, which could be a bypass route (which has two termini) (see I-210 and I-810 in image) or a radial route (known also as a beltway, beltline, or circumferential route) (see I-610 in image).]]

Auxiliary Interstate Highways are circumferential, radial, or spur highways that principally serve [[Engineering:Urban area|urban area]]s. These types of Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of its parent 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 the parent, and are given an even first digit. 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 instance, I-190 in Massachusetts is labeled north–south, while I-195 in New Jersey is labeled east–west. Some looped Interstate routes use inner–outer directions instead of compass directions, when the use of compass directions would create ambiguity. Due to the large number of these routes, auxiliary route numbers may be repeated in different states along the mainline.<ref name="fhwa_route_log">{{cite web |url = https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/index.cfm |author = Federal Highway Administration |publisher = Federal Highway Administration |date = March 22, 2007 |access-date = January 23, 2008 |title = FHWA Route Log and Finder List |archive-url = https://web.archive.org/web/20130605010643/http://www.fhwa.dot.gov/reports/routefinder/index.cfm |archive-date = June 5, 2013 |url-status = live }}</ref> Some auxiliary highways do not follow these guidelines, however.

===Alaska, Hawaii, and Puerto Rico===
[[File:Puerto Rico Interstates.svg|thumb|Map of routes in Puerto Rico that receive funding from the Interstate program, but are not signed as Interstate Highways]]
[[File:Interstate Alaska map.png|thumb|Map of routes in Alaska that receive funding from the Interstate program, but are not signed as Interstate Highways]]

The Interstate Highway System also extends to [[Earth:Alaska|Alaska]], Hawaii, and Puerto Rico, even though they have no direct land connections to any other states or territories. However, their residents still pay federal fuel and tire taxes.

The Interstates in Hawaii, all located on the most populous island of Oahu, carry the prefix '''H'''. There are three one-digit routes in the state (H-1, H-2, and H-3) and one auxiliary route (H-201). These Interstates connect several {{wipe|United States Air Force}} and naval bases together, as well as the important communities spread across Oahu, and especially within the urban core of Honolulu.

Both Alaska and Puerto Rico also have public highways that receive 90&nbsp;percent of their funding from the Interstate Highway program. The Interstates of Alaska and Puerto Rico are numbered sequentially in order of funding without regard to the rules on odd and even numbers. They also carry the prefixes '''A''' and '''PR''', respectively. However, these highways are signed according to their local designations, not their Interstate Highway numbers. Furthermore, these routes were neither planned according to nor constructed to the official Interstate Highway standards.<ref name="FHWA2" />

===Mile markers and exit numbers===
On one- or two-digit Interstates, the mile marker numbering almost always begins at the southern or western state line. If an Interstate originates within a state, the numbering begins from the location where the road begins in the south or west.  As with all guidelines for Interstate routes, however, numerous exceptions exist.

Three-digit Interstates with an even first number that form a complete circumferential (circle) bypass around a city feature mile markers that are numbered in a clockwise direction, beginning just west of an Interstate that bisects the circumferential route near a south polar location. In other words, mile marker&nbsp;1 on I-465, a {{convert|53|mi|km|adj=on}} route around Indianapolis, is just west of its junction with I-65 on the south side of Indianapolis (on the south leg of I-465), and mile marker&nbsp;53 is just east of this same junction. An exception is I-495 in the Washington metropolitan area, with mileposts increasing counterclockwise because part of that road is also part of I-95.

Most Interstate Highways use distance-based exit numbers so that the exit number is the same as the nearest mile marker.  If multiple exits occur within the same mile, letter suffixes may be appended to the numbers in alphabetical order starting with A.<ref name="INDOT-Understanding Interstate Route Numbering, Mile Markers & Exit Numbering" >{{cite web |url = http://www.in.gov/indot/2488.htm |access-date = November 26, 2011 |title = Understanding Interstate Route Numbering, Mile Markers & Exit Numbering |author = Indiana Department of Transportation |date = n.d. |publisher = Indiana Department of Transportation |archive-url = https://web.archive.org/web/20130515104711/http://www.in.gov/indot/2488.htm |archive-date = May 15, 2013 |url-status = live }}</ref>  A small number of Interstate Highways (mostly in the Northeastern United States) use sequential-based exit numbering schemes (where each exit is numbered in order starting with 1, without regard for the mile markers on the road).  One Interstate Highway, I-19 in Arizona, is signed with kilometer-based exit numbers. In the state of New York, most Interstate Highways use sequential exit numbering, with some exceptions.<ref name="Is New York State planning to change its Interstate exit numbering system from a sequential system to a distance-based milepost system?" >{{cite web |url = https://www.dot.ny.gov/about-nysdot/faq/nys-interstate-exit--system-sequential-or-milepost-system |access-date = January 1, 2003 |title = Is New York State planning to change its Interstate exit numbering system from a sequential system to a distance-based milepost system? |author = New York State Department of Transportation |date = n.d. |publisher = New York State Department of Transportation |archive-date = March 22, 2019 |archive-url = https://web.archive.org/web/20190322034420/https://www.dot.ny.gov/about-nysdot/faq/nys-interstate-exit--system-sequential-or-milepost-system |url-status = live }}</ref>

===Business routes===
{{multiple image
|align=right
|direction= horizontal
|width= 110
|image1= Business Loop 80.svg
|alt1= Business Loop Interstate 80 shield marker
|image2= Business Spur 80.svg
|alt2= Business Spur Interstate 80 shield marker
|header= Standard Interstate shields
|footer= Markers for Business Loop Interstate 80 (left) and Business Spur Interstate 80 (right)
}}
AASHTO defines a category of special routes separate from primary and auxiliary Interstate designations. These routes do not have to comply to Interstate construction or limited-access standards but are routes that may be identified and approved by the association. The same route marking policy applies to both US Numbered Highways and Interstate Highways; however, business route designations are sometimes used for Interstate Highways.<ref name="aashto_ho1">{{cite web |url = http://cms.transportation.org/sites/route/docs/HO1_Policy_Establ_Develop_USRN.pdf |title = Establishment and Development of United States Numbered Highways |date = January 2000 |access-date = January 23, 2008 |author = American Association of State Highway and Transportation Officials |publisher = American Association of State Highway and Transportation Officials |url-status = dead |archive-url = https://web.archive.org/web/20061101234239/http://cms.transportation.org/sites/route/docs/HO1_Policy_Establ_Develop_USRN.pdf |archive-date = November 1, 2006 }}</ref> Known as Business Loops and Business Spurs, these routes principally travel through the corporate limits of a city, passing through the central business district when the regular route is directed around the city. They also use a green shield instead of the red and blue shield.<ref name="aashto_ho1" /> An example would be Business Loop Interstate 75 at Pontiac, Michigan, which follows surface roads into and through downtown. Sections of BL I-75's routing had been part of US&nbsp;10 and M-24, predecessors of I-75 in the area.

==Financing==
[[File:787NorthEnd.JPG|thumb|I‑787 in Watervliet, New York, showing the exit&nbsp;8 diamond interchange]]

Interstate Highways and their rights-of-way are owned by the state in which they were built. The last federally owned portion of the Interstate System was the Woodrow Wilson Bridge on the Washington Capital Beltway. The new bridge was completed in 2009 and is collectively owned by Virginia and Maryland.<ref>{{cite web |author = Federal Highway Administration |date = n.d. |publisher = Federal Highway Administration |title = Interstate FAQ: Who owns it? |url = http://www.fhwa.dot.gov/interstate/faq.htm#question5 |access-date = March 4, 2009 |archive-url = https://web.archive.org/web/20130507121442/http://www.fhwa.dot.gov/interstate/faq.htm#question5 |archive-date = May 7, 2013 |url-status = live }}</ref> Maintenance is generally the responsibility of the state department of transportation. However, there are some segments of Interstate owned and maintained by local authorities.

About 70&nbsp;percent of the construction and maintenance costs of Interstate Highways in the United States have been paid through user fees, primarily the [[Chemistry:Fuel tax|fuel tax]]es collected by the federal, state, and local governments. To a much lesser extent they have been paid for by tolls collected on toll highways and bridges. The federal gasoline tax was first imposed in 1932 at one cent per gallon; during the Eisenhower administration, the Highway Trust Fund, established by the Highway Revenue Act in 1956, prescribed a three-cent-per-gallon fuel tax, soon increased to 4.5&nbsp;cents per gallon. Since 1993 the tax has remained at 18.4&nbsp;cents per gallon.<ref name="FHWA">{{cite web |last = Weingroff |first = Richard M. |title = When did the Federal Government begin collecting the gas tax? |url = http://www.fhwa.dot.gov/infrastructure/gastax.cfm |work = Ask the Rambler |publisher = Federal Highway Administration |access-date = June 29, 2011 |date = April 7, 2011 |archive-url = https://web.archive.org/web/20130703022304/https://www.fhwa.dot.gov/infrastructure/gastax.cfm |archive-date = July 3, 2013 |url-status = live }}</ref> Other excise taxes related to highway travel also accumulated in the Highway Trust Fund.<ref name="FHWA" /> Initially, that fund was sufficient for the federal portion of building the Interstate system, built in the early years with "10 cent dollars", from the perspective of the states, as the federal government paid 90% of the costs while the state paid 10%. The system grew more rapidly than the rate of the taxes on fuel and other aspects of driving (e. g., excise tax on tires).

The rest of the costs of these highways are borne by general fund receipts, bond issues, designated property taxes, and other taxes. The federal contribution comes overwhelmingly from motor vehicle and fuel taxes (93.5&nbsp;percent in 2007), as does about 60&nbsp;percent of the state contribution. However, any local government contributions are overwhelmingly from sources besides user fees.<ref>{{cite web |author = Federal Highway Administration |url = http://www.fhwa.dot.gov/policyinformation/statistics/2007/hf10.cfm |work = Highway Statistics 2007 |publisher = Federal Highway Administration |title = Funding For Highways and Disposition of Highway-User Revenues, All Units of Government, 2007 |date = January 3, 2012 |access-date = March 10, 2012 |archive-url = https://web.archive.org/web/20130117104528/http://www.fhwa.dot.gov/policyinformation/statistics/2007/hf10.cfm |archive-date = January 17, 2013 |url-status = live }}</ref> As decades passed in the 20th century and into the 21st century, the portion of the user fees spent on highways themselves covers about 57&nbsp;percent of their costs, with about one-sixth of the user fees being sent to other programs, including the mass transit systems in large cities. Some large sections of Interstate Highways that were planned or constructed before 1956 are still operated as toll roads, for example the Massachusetts Turnpike (I-90), the New York State Thruway (I-87 and I-90), and Kansas Turnpike (I-35, I-335, I-470, I-70). Others have had their construction bonds paid off and they have become toll-free, such as the Connecticut Turnpike (I‑95, I-395), the Richmond-Petersburg Turnpike in Virginia (also I‑95), and the Kentucky Turnpike (I‑65).

[[File:Bird's Eye 75 (31948398834).jpg|thumb|upright=0.667|A view of I-75 in Atlanta, Georgia, featuring HOV lanes running alongside the median]]
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.<ref>{{Cite news |last = Field |first = David |title = On 40th birthday, Interstates Face Expensive Midlife Crisis |work = Insight on the News |date = July 29, 1996 |pages = 40–42 |issn = 1051-4880 }}</ref> 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 (for example, the VMS company maintains I‑35 in Texas)<ref name="vmsom">{{cite web |author = VMS, Inc. |date = n.d. |url = http://www.vmsom.com/projectsoverviewbytype.shtm#Interstates |title = Projects by Type: Interstates |access-date = January 10, 2008 |publisher = VMS, Inc. |url-status = dead |archive-url = https://web.archive.org/web/20070922220101/http://www.vmsom.com/projectsoverviewbytype.shtm |archive-date = September 22, 2007 }}</ref> to meet rising costs of maintenance and allow state departments of transportation to focus on serving the fastest-growing regions in their states.

Parts of the Interstate System might have to be tolled in the future to meet maintenance and expansion demands, as has been done with adding toll [[Engineering:High-occupancy vehicle lane|HOV]]/[[Finance:High-occupancy toll lane|HOT lanes]] in cities such as Atlanta, Dallas, and {{wipe|Los Angeles}}.<!--This is not an exhaustive list, so additional examples are not needed, thanks.--> 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,<ref name="cobbrides">{{cite web |url = http://www.cobbrides.com/pdfs/1st%20toll%20project%20proposed%20for%20I.pdf |title = 1st Toll Project Proposed for I-20 East: Plan Would Add Lanes Outside I-285 |first = Ariel |last = Hart |work = The Atlanta Journal-Constitution |date = July 19, 2007 |access-date = September 27, 2007 |issn = 1539-7459 |url-status = dead |archive-url = https://web.archive.org/web/20071025045622/http://www.cobbrides.com/pdfs/1st%20toll%20project%20proposed%20for%20I.pdf |archive-date = October 25, 2007 }}</ref><ref name="atlanta_hov_future">{{cite web |url = http://www.dot.state.ga.us/preconstruction/consultantdesign/design/Future%20of%20HOV%20in%20Atlanta.pdf |archive-url = https://web.archive.org/web/20071025045621/http://www.dot.state.ga.us/preconstruction/consultantdesign/design/Future%20of%20HOV%20in%20Atlanta.pdf |url-status = dead |archive-date = October 25, 2007 |title = Future of HOV in Atlanta |first = Darryl D. |last = VanMeter |access-date = September 27, 2007 |publisher = American Society of Highway Engineers |date = October 28, 2005 }}</ref> present federal law does not allow for a state to change a freeway section to a tolled section for all traffic.{{Citation needed|date=March 2009}}

===Tolls===
{{Category see also|Tolled sections of Interstate Highways}}
[[File:2022-07-31 12 16 28 View west along Interstate 76 (Pennsylvania Turnpike Philadelphia Extension) just west of Exit 320 in Charlestown Township, Chester County, Pennsylvania.jpg|thumb|right|An I-76 trailblazer along the Pennsylvania Turnpike with the black-on-yellow "Toll" sign]]

About {{convert|2,900|mi|km}} of toll roads are included in the Interstate Highway System.<ref>{{cite web |first = Martin H. |last = Weiss |date = April 7, 2011 |url = http://www.fhwa.dot.gov/highwayhistory/howmany.cfm |title = How Many Interstate Programs Were There? |work = Highway History |publisher = Federal Highway Administration |access-date = March 10, 2012 |archive-url = https://web.archive.org/web/20130607213511/http://www.fhwa.dot.gov/highwayhistory/howmany.cfm |archive-date = June 7, 2013 |url-status = live }}</ref> 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. Thus, these toll roads were [[Social:Grandfather clause|grandfathered]] into the Interstate Highway System.<ref>{{cite web |last = Weingroff |first = Richard F. |date = August 2, 2011 |url = http://www.fhwa.dot.gov/infrastructure/tollroad.cfm |title = Why Does The Interstate System Include Toll Facilities? |work = Ask the Rambler |publisher = Federal Highway Administration |access-date = March 10, 2012 |archive-url = https://web.archive.org/web/20130518082124/http://www.fhwa.dot.gov/infrastructure/tollroad.cfm |archive-date = May 18, 2013 |url-status = live }}</ref>

Toll roads designated as Interstates (such as the Massachusetts Turnpike) were typically allowed to continue collecting tolls, but are generally ineligible to receive federal funds for maintenance and improvements. Some toll roads that did receive federal funds to finance emergency repairs (notably the Connecticut Turnpike (I-95) following the Mianus River Bridge collapse) were required to remove tolls as soon as the highway's construction bonds were paid off. 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 I-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 I-376 included a section of Pennsylvania Route&nbsp;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 ''[[Engineering:Manual on Uniform Traffic Control Devices|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.<ref>{{cite book |chapter-url = http://mutcd.fhwa.dot.gov/shsm_interim/index.htm |chapter = Interim Releases for New and Revised Signs |publisher = Federal Highway Administration |date = November 16, 2011 |title = Standard Highway Signs and Markings |access-date = March 10, 2012 |author = Federal Highway Administration |archive-date = March 18, 2012 |archive-url = https://web.archive.org/web/20120318045005/http://mutcd.fhwa.dot.gov/shsm_interim/index.htm |url-status = live }}</ref>

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 [[Finance:Public–private partnership|public–private partnership]]s. 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.  Many states have started using [[Finance:High-occupancy toll lane|High-occupancy toll lane]] and other partial tolling methods, whereby certain lanes of highly congested freeways are tolled, while others are left free, allowing people to pay a fee to travel in less congested lanes.  Examples of recent projects to add HOT lanes to existing freeways include the Virginia HOT lanes on the Virginia portions of the Capital Beltway and other related interstate highways (I-95, I-495, I-395) and the addition of express toll lanes to Interstate 77 in North Carolina in the Charlotte metropolitan area.

===Chargeable and non-chargeable Interstate routes===
Interstate Highways financed with federal funds are known as "chargeable" Interstate routes, and are considered part of the {{convert|42000|mi|km|adj=on}} network of highways. Federal laws also allow "non-chargeable" Interstate routes, highways funded similarly to state and US Highways to be signed as Interstates, if they both meet the Interstate Highway standards and are logical additions or connections to the system.<ref>{{usc|23|103(c)}}, Interstate System.</ref><ref>{{USPL|99|599|Surface Transportation Assistance Act of 1978}}</ref> 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.<ref name="FHWA2">{{cite web |first = Tony |last = DeSimone |url = http://www.fhwa.dot.gov/reports/routefinder/#s06 |title = FHWA Route Log and Finder List: Additional Designations |publisher = Federal Highway Administration |date = March 22, 2007 |access-date = January 4, 2010 |archive-url = https://web.archive.org/web/20140805032748/http://www.fhwa.dot.gov/reports/routefinder/#s06 |archive-date = August 5, 2014 |url-status = live }}</ref>

==Signage==

===Interstate shield===
[[File:Texas interstate shield proposals.jpg|thumb|Several Interstate shield design proposals submitted by the Texas Highway Department|alt=Three black and white submissions, the third being similar to the modern Interstate Highway shield]]

Interstate Highways are signed by a number placed on a red, white, and blue sign. The shield design itself is a registered trademark of the American Association of State Highway and Transportation Officials.<ref>{{cite web |url = http://tsdr.uspto.gov/#caseNumber=72239199&caseType=SERIAL_NO&searchType=statusSearch |author = American Association of State Highway Officials |date = September 19, 1967 |title = Trademark Registration 0835635 |work = Trademark Electronic Search System |publisher = United States Patent and Trademark Office |access-date = April 27, 2014 |archive-url = https://web.archive.org/web/20130502074700/http://tsdr.uspto.gov/#caseNumber=72239199&caseType=SERIAL_NO&searchType=statusSearch |archive-date = May 2, 2013 |url-status = dead }}</ref> The colors red, white, and blue were chosen because they are the colors of the American flag. 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, allowing for the printing of larger and more-legible digits. Signs with the shield alone are placed periodically throughout each Interstate as reassurance markers. These signs usually measure {{convert|36|in|cm}} high, and are {{convert|36|in|cm}} wide for two-digit Interstates or {{convert|45|in|cm}} for three-digit Interstates.<ref name="SHS">{{Cite book |author = Federal Highway Administration |chapter-url = http://mutcd.fhwa.dot.gov/SHSe/Guide.pdf |chapter = Guide Signs |title = Standard Highway Signs |edition = 2004 English |location = Washington, DC |publisher = Federal Highway Administration |oclc = 69678912 |orig-year = 2004 |date = May 10, 2005 |at = pp. 3-1 to 3-3 |access-date = February 22, 2012 |archive-date = February 5, 2012 |archive-url = https://web.archive.org/web/20120205020037/http://mutcd.fhwa.dot.gov/SHSe/Guide.pdf |url-status = live }}</ref>

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.<ref name="SHS" /> The green shield is employed to mark the main route through a city's central business district, which intersects the associated Interstate 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.<ref name="MUTCD2D">{{cite book |author = Federal Highway Administration |date = December 2009 |chapter = Chapter 2D. Guide Signs: Conventional Roads |title = Manual on Uniform Traffic Control Devices |chapter-url = http://mutcd.fhwa.dot.gov/pdfs/2009/part2d.pdf |page = 142 |location = Washington, DC |publisher = Federal Highway Administration |oclc = 496147812 |edition = 2009 |title-link = Manual on Uniform Traffic Control Devices |access-date = February 22, 2012 |archive-date = March 15, 2012 |archive-url = https://web.archive.org/web/20120315132420/http://mutcd.fhwa.dot.gov/pdfs/2009/part2d.pdf |url-status = live }}</ref> 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.<ref name="MDOT11">{{cite MDOT map |year= 2011 |inset= Lansing |link= yes }}</ref>

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 that included 100&nbsp;entries;<ref>{{cite magazine |author = Texas Transportation Institute |url = http://tti.tamu.edu/publications/researcher/v41n4/41_4.pdf |title = Ties to Texas |magazine = Texas Transportation Researcher |volume = 41 |issue = 4 |pages = 20–21 |year = 2005 |archive-url = https://web.archive.org/web/20100820082038/http://tti.tamu.edu/publications/researcher/v41n4/41_4.pdf |archive-date = August 20, 2010 |author-link = Texas Transportation Institute }}</ref><ref>{{cite web |author = American Association of State Highway and Transportation Officials |year = 2006 |url = http://www.interstate50th.org/gallery.shtml |title = Image Gallery |work = The Interstate is 50 |publisher = American Association of State Highway and Transportation Officials |access-date = February 22, 2012 |url-status = dead |archive-url = https://web.archive.org/web/20120225170530/http://www.interstate50th.org/gallery.shtml |archive-date = February 25, 2012 }}</ref> at the time, the shield color was a dark navy blue and only {{convert|17|in|cm}} wide.<ref>{{cite book |author = American Association of State Highway Officials |year = 1958 |title = Manual for Signing and Pavement Marking of the National System of Interstate and Defense Highways |location = Washington, DC |publisher = American Association of State Highway Officials |oclc = 3332302 }}</ref> The ''[[Engineering:Manual on Uniform Traffic Control Devices|Manual on Uniform Traffic Control Devices]]'' (MUTCD) standards revised the shield in the 1961,<ref name="MUTDC61">{{cite book |author1 = National Joint Committee on Uniform Traffic Control Devices |author2 = American Association of State Highway Officials |year = 1961 |chapter = Part&nbsp;1: Signs |chapter-url = http://www.trafficsign.us/oldmutcd/1961/1-signs.pdf |title = Manual on Uniform Traffic Control Devices for Streets and Highways |edition = 1961 |location = Washington, DC |publisher = Bureau of Public Roads |pages = 79–80 |oclc = 35841771 |access-date = February 22, 2012 |archive-date = April 14, 2012 |archive-url = https://web.archive.org/web/20120414190819/http://www.trafficsign.us/oldmutcd/1961/1-signs.pdf |url-status = live }}</ref> 1971,<ref name="MUTDC71">{{cite book |author1 = National Joint Committee on Uniform Traffic Control Devices |author2 = American Association of State Highway Officials |year = 1971 |chapter = Chapter&nbsp; 2D. Guide Signs: Conventional Roads |chapter-url = http://www.trafficsign.us/oldmutcd/1971/2d-guidesignsconv.pdf |title = Manual on Uniform Traffic Control Devices for Streets and Highways |edition = 1971 |location = Washington, DC |publisher = Federal Highway Administration |page = 88 |oclc = 221570 |access-date = February 22, 2012 |archive-date = November 29, 2011 |archive-url = https://web.archive.org/web/20111129185358/http://trafficsign.us/oldmutcd/1971/2d-guidesignsconv.pdf |url-status = live }}</ref> and 1978<ref name="MUTDC78">{{cite book |author = National Advisory Committee on Uniform Traffic Control Devices |year = 1978 |chapter = Chapter&nbsp; 2D. Guide Signs: Conventional Roads |chapter-url = http://www.trafficsign.us/oldmutcd/1978/2d-guidesigns.pdf |title = Manual on Uniform Traffic Control Devices for Streets and Highways |edition = 1978 |location = Washington, DC |publisher = Federal Highway Administration |page = ((2D-5)) |oclc = 23043094 |access-date = February 22, 2012 |archive-date = November 29, 2011 |archive-url = https://web.archive.org/web/20111129165921/http://trafficsign.us/oldmutcd/1978/2d-guidesigns.pdf |url-status = live }}</ref> editions.

===Exit numbering===
The majority of Interstates have exit numbers. Like other highways, Interstates feature guide signs that list control cities to help direct drivers through interchanges and exits toward their desired destination. All traffic signs and [[Engineering:Lane|lane]] markings on the Interstates are supposed to be designed in compliance with the [[Engineering:Manual on Uniform Traffic Control Devices|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. To control costs, California began to incorporate exit numbers on its freeways in 2002—Interstate, US, and state routes alike. Caltrans commonly installs exit number signage only when a freeway or interchange is built, reconstructed, retrofitted, or repaired, and it is usually tacked onto the top-right corner of an already existing sign. Newer signs along the freeways follow this practice as well. Most exits along California's Interstates now have exit number signage, particularly in rural areas. California, however, still does not use mileposts, although a few exist for experiments or for special purposes.<ref>{{cite web |first = Daniel P. |last = Faigin |date = December 29, 2015 |url = http://www.cahighways.org/num-postmiles.html |work = California Highways |title = Numbering Conventions: Post Miles |access-date = March 15, 2017 |archive-url = https://web.archive.org/web/20170131190248/http://www.cahighways.org/num-postmiles.html |archive-date = January 31, 2017 |url-status = live }}{{self-published source|date=March 2017}}</ref>{{self-published inline|certain=y|date=March 2017}} 

In 2010–2011, the Illinois State Toll Highway Authority posted all new mile 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. This also applied to the tolled portion of the Ronald Reagan Tollway (I-88). The tollway also added exit number tabs to the exits.{{citation needed|date=December 2010}}

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|metric system]] of measurement had gained enough traction that it was mistakenly assumed that all highway measurements would eventually be changed to metric (and some distance signs retain metric distances);<ref>{{cite web |last1 = Zhang |first1 = Sarah |date = October 7, 2014 |title = An Arizona Highway Has Used the Metric System Since the 80s |url = https://gizmodo.com/an-arizona-highway-has-used-the-metric-system-since-the-1643536691 |website = Gizmodo |access-date = February 25, 2019 |archive-url = https://web.archive.org/web/20190225103128/https://gizmodo.com/an-arizona-highway-has-used-the-metric-system-since-the-1643536691 |archive-date = February 25, 2019 |url-status = live}}</ref> 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 highways, including the New York State Thruway, use sequential exit-numbering schemes. Exits on the New York State Thruway count up from Yonkers traveling north, and then west from Albany. I‑87 in New York State is numbered in three sections. The first section makes up the Major Deegan Expressway in the Bronx, with interchanges numbered sequentially from 1 to 14. The second section of I‑87 is a part of the New York State Thruway that starts in Yonkers (exit&nbsp;1) and continues north to Albany (exit&nbsp;24); at Albany, the Thruway turns west and becomes I‑90 for exits&nbsp;25 to 61. From Albany north to the Canadian border, the exits on I‑87 are numbered sequentially from 1 to 44 along the Adirondack Northway. This often leads to confusion as there is more than one exit on I‑87 with the same number. For example, exit&nbsp;4 on Thruway section of I‑87 connects with the Cross County Parkway in Yonkers, but exit&nbsp;4 on the Northway is the exit for the Albany airport. These two exits share a number but are located {{convert|150|mi}} apart.

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, New Hampshire, New York, and Vermont; as such, three of the main Interstate Highways that remain completely within these states (87, 88, 89) have interchanges numbered sequentially along their entire routes. Maine, Massachusetts, Pennsylvania, Virginia, Georgia, and Florida followed this system for a number of years, but have since converted to mileage-based exit numbers. Georgia renumbered in 2000, while Maine did so in 2004. Massachusetts converted their exit numbers in 2021, and most recently-Rhode Island in 2022.<ref>{{cite web |author = Massachusetts Department of Transportation |date = n.d. |title = Massachusetts Department of Transportation completed projects |url = https://www.mass.gov/info-details/massachusetts-department-of-transportation-completed-projects#statewide-exit-renumbering-project---2021- |publisher = Massachusetts Department of Transportation |access-date = September 20, 2022 }}</ref> 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 that are signed as I‑95 and I‑78, also has sequential numbering, but other Interstates within New Jersey use mile markers.

===Sign locations===
There are four common signage methods on Interstates:
* Locating a sign on the ground to the side of the highway, mostly the right, and is used to denote exits, as well as [[Engineering:Rest area|rest area]]s, motorist services such as gas and lodging, recreational sites, and freeway names
* Attaching the sign to an overpass
* Mounting on full gantries that bridge the entire width of the highway and often show two or more signs
* Mounting on half-gantries that are located on one side of the highway, like a ground-mounted sign

==Statistics==

===Volume===
* '''Heaviest traveled''': 379,000&nbsp;vehicles per day: I-405 in Los Angeles, California (2011 estimate).<ref>{{cite press release |first = Doug |last = Hecox |title = New FHWA Report Reveals States with the Busiest Highways |url = https://www.transportation.gov/briefing-room/new-fhwa-report-reveals-states-busiest-highways |publisher = Federal Highway Administration |access-date = August 30, 2022 |date = August 1, 2019 }}</ref>

===Elevation===
* '''Highest''': {{convert|11158|ft|m}}: I-70 in the Eisenhower Tunnel at the Continental Divide in the Colorado Rocky Mountains.<ref name="Interstate Highway System Fact Sheet">{{cite web |url = http://www.interstate50th.org/docs/InterstateHighwayFactSheet.pdf |archive-url = https://web.archive.org/web/20081010060935/http://www.interstate50th.org/docs/InterstateHighwayFactSheet.pdf |archive-date = October 10, 2008 |title = Interstate Highway Fact Sheet |publisher = American Association of State Highway and Transportation Officials |access-date = February 22, 2012 |author = American Association of State Highway and Transportation Officials |date = n.d. }}</ref>
* '''Lowest (land)''': {{convert|−52|ft|m|}}: I-8 at the New River near Seeley, California.<ref name="Interstate Highway System Fact Sheet" />
* '''Lowest (underwater)''': {{convert|-103|ft|m}}: I-95 in the Fort McHenry Tunnel under the Baltimore Inner Harbor.<ref>{{cite web |first1 = Jerry |last1 = Hall |first2 = Loretta |last2 = Hall |name-list-style = amp |date = July 1, 2009 |url = http://westernite.org/2009/the-adobe-tower-interesting-items-about-the-interstate-system/ |title = The Adobe Tower: Interesting Items about the Interstate System |work = Westernite |publisher = Western District of the Institute of Transportation Engineers |access-date = July 23, 2013 |url-status = dead |archive-url = https://web.archive.org/web/20130928111912/http://westernite.org/2009/the-adobe-tower-interesting-items-about-the-interstate-system/ |archive-date = September 28, 2013 }}</ref>

===Length===
* '''Longest (east–west)''': {{convert|3020.54|mi|km}}: I-90 from Boston, Massachusetts, to Seattle, Washington.<ref name="fhwa-facts-1">{{cite web |first1 = Jon |last1 = Obenberger |first2 = Tony |last2 = DeSimone |name-list-style = amp |url = http://www.fhwa.dot.gov/programadmin/interstate.cfm#interstate_trivia |title = Interstate System Facts |work = Route Log and Finder List |publisher = Federal Highway Administration |date = April 7, 2011 |access-date = February 22, 2012 |archive-url = https://web.archive.org/web/20130713093446/http://www.fhwa.dot.gov/programadmin/interstate.cfm#interstate_trivia |archive-date = July 13, 2013 |url-status = live }}</ref><ref name="fhwa-facts-2">{{cite web |author = Federal Highway Administration |url = http://www.fhwa.dot.gov/reports/routefinder/#s11 |title = Miscellaneous Interstate System Facts |publisher = Federal Highway Administration |date = April 6, 2011 |access-date = February 22, 2012 |archive-url = https://web.archive.org/web/20140805032748/http://www.fhwa.dot.gov/reports/routefinder/#s11 |archive-date = August 5, 2014 |url-status = live }}</ref>
* '''Longest (north–south)''': {{convert|1908|mi|km|abbr=on}}: I-95 from the Canadian border near Houlton, Maine, to Miami, Florida.<ref name="fhwa-facts-1" /><ref name="sofield" /> 
* '''Shortest (two-digit)''': {{convert|1.40|mi|km|abbr=on}}: I-69W in Laredo, Texas.<ref>{{cite web |title = FHWA Route Log and Finder List |url = https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/ |date = January 31, 2018 |publisher = Federal Highway Administration |access-date = August 25, 2018 |archive-url = https://web.archive.org/web/20180711022000/https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/ |archive-date = July 11, 2018 |url-status = live }}</ref>
* '''Shortest (auxiliary)''': {{convert|0.70|mi|km|abbr=on}}: I-878 in Queens, {{wipe|New York City}}, New York.<ref>{{cite web |first = Kevin |last = Adderly |date = December 31, 2016 |title = Table 2: Auxiliary Routes of the Dwight D. Eisenhower National System of Interstate and Defense Highways as of December 31, 2016 |url = https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/table02.cfm |work = Route Log and Finder List |publisher = Federal Highway Administration |access-date = September 24, 2017 }}</ref><ref>{{Cite news |url = http://articles.latimes.com/1996-04-19/local/me-60325_1_freeway-system |title = The Freeway Numbers Game |last = Curtiss |first = Aaron |date = April 19, 1996 |work = Los Angeles Times |access-date = August 31, 2017 |issn = 0458-3035 }}</ref>
* '''Longest segment within state lines''': {{convert|877|mi|km|abbr=on}}: I-10 in Texas from the New Mexico state line near El Paso to the Louisiana state line near Orange, Texas.<ref>{{TxDOT|IH|10|access-date=August 31, 2010}}</ref>
* '''Shortest segment within state lines''': {{convert|453|ft|m|abbr=on}}: I-95/I-495 (Capital Beltway) on the Woodrow Wilson Bridge across the Potomac River where they briefly cross the southernmost tip of the District of Columbia between its borders with Maryland and Virginia.<ref name="fhwa-facts-2" />
* '''Longest concurrency''': {{convert|278.4|mi|km|abbr=on}}: I-80 and I-90; Gary, Indiana, to Elyria, Ohio.<ref name="route-finder">{{cite web |first = Jeff |last = Price |url = https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/table01.cfm |title = Table 1: Main Routes of the Dwight D. Eisenhower National System Of Interstate and Defense Highways as of December 31, 2018 |work = Route Log and Finder List |publisher = Federal Highway Administration |date = May 6, 2019 |access-date = June 21, 2021 |archive-url = https://web.archive.org/web/20120422220808/http://www.fhwa.dot.gov/reports/routefinder/table1.cfm |archive-date = April 22, 2012 |url-status = live }}</ref>
<!--* '''Shortest concurrency''': {{convert|.22|mi|km|abbr=on}}: I-71 and I-75; in Cincinnati from the Ohio River on the lower deck of the Brent Spence Bridge at the state line north to the Fort Washington Way entrance west of Paul Brown Stadium.<ref name="route-finder" />--><!--I'm commenting out this entire entry because the concurrency continues for 19.70 more miles in Kentucky according to the source, so the full I-71/I-75 concurrency is almost 20 miles long...-->

===States===
* '''Most states served by an Interstate''': 15&nbsp;states plus the District of Columbia: I-95 through Florida, Georgia, South Carolina, North Carolina, Virginia, DC, Maryland, Delaware, Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, and Maine.<ref name="fhwa-facts-1" />
* '''Most Interstates in a state''': 32 routes: New York, totaling {{convert|1750.66|mi|km|abbr=on}}<ref name="fhwa route log">{{cite web |title = Table 3: Interstate Routes in Each of the 50 States, District of Columbia, and Puerto Rico |url = https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/table03.cfm |work = Route Log and Finder List |publisher = Federal Highway Administration |access-date = August 25, 2018 |archive-url = https://web.archive.org/web/20180711030748/https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/table03.cfm |archive-date = July 11, 2018 |url-status = live }}</ref>
* '''Most primary Interstates in a state''': 13 routes: Illinois{{#tag:ref|This counts the suffixed routes in Texas (I-35E, I-35W, I-69E, I-69C, and I-69W) as auxiliary routes or parts of the same primary Interstate and not separate primary Interstates.|group=lower-alpha}}<ref name="fhwa route log" />
* '''Most Interstate mileage in a state''': {{convert|3233.45|mi|km|abbr=on}}: Texas, in 17 different routes.<ref name="fhwa-facts-1" />
* '''Fewest Interstates in a state''': 3 routes: Delaware, New Mexico, North Dakota, Puerto Rico, and Rhode Island.<ref name="fhwa route log" />
* '''Fewest primary Interstates in a state''': 1 route: Delaware, Maine, and Rhode Island (I-95 in each case).<ref name="fhwa route log" />
* '''Least Interstate mileage in a state''': {{convert|40.61|mi|km|abbr=on}}: Delaware, in 3 different routes.<ref name="fhwa route log" />

==Impact and reception==

Following the passage of the Federal Aid Highway Act of 1956, passenger rail declined sharply as did freight rail for a short time, but the trucking industry expanded dramatically and the cost of shipping and travel fell sharply.<ref>{{Cite web |date = July 20, 2017 |title = American Railroads |url = https://americanhistory.si.edu/america-on-the-move/essays/american-railroads |access-date = October 2, 2023 |website = National Museum of American History |language = en }}</ref>{{Citation needed|reason=Need source for costs of shipping and travel|date=October 2023}} [[Engineering:Suburbanization|Suburbanization]] became possible, with the rapid growth of larger, sprawling, and more car dependent housing than was available in central cities, enabling [[Social:Racial segregation|racial segregation]] by white flight.<ref name=":0">{{Cite news |last = Kruse |first = Kevin M. |date = August 14, 2019 |title = How Segregation Caused Your Traffic Jam |work = {{wipe|The New York Times}} |url = https://www.nytimes.com/interactive/2019/08/14/magazine/traffic-atlanta-segregation.html |issn = 0362-4331 }}</ref><ref>{{Cite web |url = https://www.kcrw.com/news/shows/greater-la/robert-fuller-freeways-urbanism-race/la-freeways |title = How freeways represent the racial divide in LA |date = June 30, 2020 |website = KCRW |last = Chiotakis |first = Steve }}</ref><ref>{{Cite journal |title = Highways and segregation |first = Avichal |last = Mahajan |date = June 26, 2023 |journal = Journal of Urban Economics |pages = 103574 |doi = 10.1016/j.jue.2023.103574 |s2cid = 259681981 |doi-access = free }}</ref> A sense of isolationism developed in suburbs, with suburbanites wanting to keep urban areas disconnected from the suburbs.<ref name=":0" /> Tourism dramatically expanded, creating a demand for more service stations, motels, restaurants and visitor attractions. The Interstate System was the basis for urban expansion in the Sun Belt, and many urban areas in the region are thus very car-dependent.<ref name=":1">{{Cite book |series = History in Dispute |volume = 2 |title = American Social and Political Movements, 1945–2000: Pursuit of Liberty |date = 2000 |publisher = St. James Press |isbn = 978-1-55862-396-5 |editor-last = Allison |editor-first = Robert J. |location = Detroit }}{{page needed|date=December 2023}}</ref> The highways may have contributed to increased economic productivity in, and thereby increased migration to, the [[Astronomy:Sun Belt|Sun Belt]].<ref>{{cite book |type = Report |last1 = Glaeser |first1 = Edward L. |last2 = Tobio |first2 = Kristina |title = The Rise of the Sunbelt |year = 2007 |publisher = Taubman Center Policy Briefs |id = PB-2007-5 }}</ref> In rural areas, towns and small cities off the grid lost out as shoppers followed the interstate and new factories were located near them.<ref>{{cite magazine |last = Blas |first = Elisheva |title = The Dwight D. Eisenhower National System of Interstate and Defense Highways: The Road to Success? |url = http://www.societyforhistoryeducation.org/pdfs/N10_NHD_Blas_Junior.pdf |volume = 44 |issue = 1 |date = November 2010 |pages = 127–142 |magazine = The History Teacher |publisher = Society for History Education |location = Long Beach, California |issn = 0018-2745 |access-date = April 27, 2017 |archive-url = https://web.archive.org/web/20170402233423/http://www.societyforhistoryeducation.org/pdfs/N10_NHD_Blas_Junior.pdf |archive-date = April 2, 2017 |url-status = live |jstor = 25799401 }}</ref>

The system had a profound effect on interstate shipping. The Interstate Highway System was being constructed at the same time as the [[Engineering:Intermodal container|intermodal shipping container]] made its debut. These containers could be placed on trailers behind trucks and shipped across the country with ease. A new road network and shipping containers that could be easily moved from ship to train to truck, meant that overseas manufacturers and domestic startups could get their products to market quicker than ever, allowing for accelerated economic growth.<ref name="fox1"/> Forty years after its construction, the Interstate Highway system returned on investment, making $6 for every $1 spent on the project.<ref name="Cox & Love">{{cite book |last1 = Cox |first1 = Wendell |last2 = Jean |first2 = Love |date = June 1996 |title = 40 Years of the US Interstate Highway System: An Analysis The Best Investment A Nation Ever Made |url = http://www.publicpurpose.com/freeway1.htm |publisher = American Highway Users Alliance |via = Public Purpose |access-date = November 21, 2022 }}</ref>

The system had a particularly strong effect in Southern states, where major highways were inadequate{{Citation needed|date=March 2023}}. The new system facilitated the relocation of heavy manufacturing to the South and spurred the development of Southern-based corporations like [[Company:Walmart|Walmart]] (in Arkansas) and [[Company:FedEx|FedEx]] (in Tennessee).<ref name="fox1">{{cite news |last1 = Fox |first1 = Justin |title = The Great Paving: How the Interstate Highway System Helped Create the Modern Economy—and Reshaped the Fortune 500 |url = http://archive.fortune.com/magazines/fortune/fortune_archive/2004/01/26/358835/index.htm |work = Fortune |date = January 26, 2004 |access-date = May 10, 2019 |archive-url = https://web.archive.org/web/20180601030412/http://archive.fortune.com/magazines/fortune/fortune_archive/2004/01/26/358835/index.htm |archive-date = June 1, 2018 |url-status = live }}</ref>

The Interstate Highway System also dramatically affected American culture, contributing to cars becoming more central to the American identity. Before, driving was considered an excursion that required some amount of skill and could have some chance of unpredictability. With the standardization of signs, road widths and rules, these unpredictabilities became a thing of the past. Justin Fox wrote, "By making road more reliable and by making Americans more reliant on them, they took away most of the adventure and romance associated with driving."<ref name="fox1"/>

The Interstate Highway System has been criticized for contributing to the decline of some cities that were divided by Interstates, and for displacing minority neighborhoods in urban centers.<ref>{{cite news |last1 = Stromberg |first1 = Joseph |title = Highways Gutted American Cities. So Why Did They Build Them? |url = https://www.vox.com/2015/5/14/8605917/highways-interstate-cities-history |work = Vox |date = May 11, 2016 |access-date = May 10, 2019 |archive-url = https://web.archive.org/web/20190425175726/https://www.vox.com/2015/5/14/8605917/highways-interstate-cities-history |archive-date = April 25, 2019 |url-status = live }}</ref> Between 1957 and 1977, the Interstate System alone displaced over 475,000 households and one million people across the country.<ref>{{cite news |url = https://www.nbcnews.com/specials/america-highways-inequality/ |title = Bulldozed and bisected: Highway construction built a legacy of inequality |date = June 18, 2021 |first1 = Suzanne |last1 = Gamboa |first2 = Phil |last2 = McCausland |first3 = Josh |last3 = Lederman |first4 = Ben |last4 = Popken |publisher = NBC News |access-date = June 18, 2023 }}</ref> Highways have also been criticized for increasing racial segregation by creating physical barriers between neighborhoods,<ref>{{Cite news |last = Miller |first = Johnny |date = February 21, 2018 |title = Roads to Nowhere: How Infrastructure Built on American Inequality |url = http://www.theguardian.com/cities/2018/feb/21/roads-nowhere-infrastructure-american-inequality |access-date = April 3, 2021 |website = The Guardian |location = London |language = en |archive-date = April 4, 2021 |archive-url = https://web.archive.org/web/20210404202301/https://www.theguardian.com/cities/2018/feb/21/roads-nowhere-infrastructure-american-inequality |url-status = live }}</ref> and for overall reductions in available housing and population in neighborhoods affected by highway construction.<ref>{{cite magazine |last1 = Nall |first1 = Clayton |last2 = O'Keeffe |first2 = Zachary P. |title = What Did Interstate Highways Do to Urban Neighborhoods? |magazine = What Did Interstate Highways Do to Urban Neighborhoods? |date = 2018 |page = 30 |url = http://www.nallresearch.com/uploads/7/9/1/7/7917910/urbanhighways.pdf |access-date = March 17, 2022 |archive-date = April 3, 2021 |archive-url = https://web.archive.org/web/20210403042929/http://www.nallresearch.com/uploads/7/9/1/7/7917910/urbanhighways.pdf |url-status = live }}</ref>  Other critics have blamed the Interstate Highway System for the decline of public transportation in the United States since the 1950s,<ref>{{cite news |last1 = Stromberg |first1 = Joseph |title = The Real Reason American Public Transportation Is Such a Disaster |url = https://www.vox.com/2015/8/10/9118199/public-transportation-subway-buses |work = Vox |date = August 10, 2015 |access-date = May 10, 2019 |archive-url = https://web.archive.org/web/20190510182647/https://www.vox.com/2015/8/10/9118199/public-transportation-subway-buses |archive-date = May 10, 2019 |url-status = live }}</ref> of which minorities and low-income residents are 3 to 6 times more likely to use.<ref name=":2">{{Cite web |last1 = Fitzgerald |first1 = Joan |last2 = Agyeman |first2 = Julian |date = September 7, 2021 |title = Removing urban highways can improve neighborhoods blighted by decades of racist policies |url = http://theconversation.com/removing-urban-highways-can-improve-neighborhoods-blighted-by-decades-of-racist-policies-166220 |access-date = December 4, 2023 |website = The Conversation |language = en-US }}</ref> The Interstate System has also contributed to continued resistance against new public transportation.<ref name=":0" />

The Interstate Highway System had a negative impact on minority groups, especially in urban areas. Even though the government used eminent domain to obtain land for the Interstates, it was still economical to build where land was cheapest. This cheap land was often located in predominately minority areas.<ref name=":1" /> Not only were minority neighborhoods destroyed, but in some cities the Interstates were used to divide white and minority neighborhoods.<ref name=":0" /> These practices were common in cities both in the North and South, including Nashville, Miami, {{wipe|Chicago}}, Detroit, and many other cities. The division and destruction of neighborhoods led to the limitation of employment and other opportunities, which deteriorated the economic fabric of neighborhoods.<ref name=":2" /> Neighborhoods bordering Interstates have a much higher level of particulate air pollution and are more likely to be chosen for polluting industrial facilities.<ref name=":2" />

==See also==
* Highway systems by country
* List of controlled-access highway systems
* Non-motorized access on freeways

== Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist}}

==Further reading==
{{refbegin}}
* {{cite magazine |last = Arcadi |first = Teal |year = 2022 |title = Partisanship and Permanence: How Congress Contested the Origins of the Interstate Highway System and the Future of American Infrastructure |magazine = Modern American History |volume = 5 |pages = 53–77 |doi = 10.1017/mah.2022.4 |doi-access = free }}
* {{cite book |last = Browning |first = Edgar A |year = 2011 |title = Roadbuilding Construction Equipment at Work: Building the Interstate Highways through New England's Green Mountains |publisher = Icongrafix |isbn = 978-1-58388-277-1 }}
* {{cite book |last = Friedlaender |first = Ann Fetter |year = 1965 |title = The Interstate Highway System. A Study in Public Investment |url = https://archive.org/details/interstatehighwa0000frie |url-access = registration |location = Amsterdam |publisher = North-Holland Publishing |oclc = 498010 }}
* {{cite book |last = Hanlon |first = Martin D. |year = 1997 |title = You Can Get There from Here: How the Interstate Highways Transformed America |location = New York |publisher = Basingstoke |isbn = 978-0-312-12909-5 }}
* {{cite book |last = Lewis |first = Tom |year = 1997 |title = Divided Highways: Building the Interstate Highways, Transforming American Life |location = New York |publisher = Viking |isbn = 978-0-670-86627-4 |url-access = registration |url = https://archive.org/details/dividedhighwaysb00lewi }}
* {{cite magazine |last1 = Lichter |first1 = Daniel T. |first2 = Glenn V. |last2 = Fuguitt |title = Demographic Response to Transportation Innovation: The Case of the Interstate Highway |magazine = Social Forces |volume = 59 |issue = 2 |date = December 1980 |pages = 492–512 |jstor = 2578033 |doi = 10.1093/sf/59.2.492 }}
* {{cite book |last = Rose |first = Mark H. |year = 1990 |title = Interstate: Express Highway Politics 1939–1989 |location = Knoxville |publisher = University of Tennessee Press |isbn = 978-0-87049-671-4 }}
{{refend}}

==External links==
* {{osmrelation|9714600}}
* [http://www.fhwa.dot.gov/programadmin/interstate.cfm Dwight D. Eisenhower National System of Interstate and Defense Highways], Federal Highway Administration (FHWA)
* [https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/index.cfm Route Log and Finder List], FHWA
* [http://www.fhwa.dot.gov/research/tfhrc/ Turner-Fairbank Highway Research Center], FHWA
* [https://www.eisenhowerlibrary.gov/research/online-documents/interstate-highway-system Interstate Highway System], Dwight D. Eisenhower Presidential Library and Museum
* [https://www.pbs.org/now/shows/535/index.html "Keep on Trucking?: Would you pay more in taxes to fix roads and rail?"], ''NOW'' on PBS


[[Category:Transport systems]]

{{Sourceattribution|Interstate Highway System}}