Editing Interstate Highway System (section)

===Primary (one- and two-digit) Interstates===
<!--Interstate 50 re-directs to this section. If this section is renamed, please update the re-direct.-->
{{See also|List of Interstate Highways}}
[[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|[[Interstate 78 in Pennsylvania|I‑78]] and {{nowrap|[[U.S. Route 22 in Pennsylvania|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 |author-link = American Association of State Highway and Transportation Officials }}</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, [[Interstate 294|I-294]] is a loop that connects at both ends to [[Interstate 94|I-94]], while [[Interstate 787|I-787]] is a short spur route attached to [[Interstate 87 (New York)|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 [[United States Numbered Highway System|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 = [[Los Angeles Times]] |url = https://www.latimes.com/archives/la-xpm-2001-nov-13-me-3653-story.html |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 = https://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 [[Interstate 5|I-5]] between Canada and Mexico along the [[West Coast of the United States|West Coast]] to [[Interstate 95|I‑95]] between Canada and [[Miami, Florida]] along the [[East Coast of the United States|East Coast]]. Major west–east arterial Interstates increase in number from [[Interstate 10|I-10]] between [[Santa Monica, California]], and [[Jacksonville, Florida]], to [[Interstate 90|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 = https://highways.dot.gov/highway-history/interstate-system/50th-anniversary/interstate-frequently-asked-questions |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 [[Interstate 69|I-69]] and [[Interstate 74|I-74]]) and some just happen to share route designations (such as [[Interstate 76 (disambiguation)|I-76]], [[Interstate 84 (disambiguation)|I-84]], [[Interstate 86 (disambiguation)|I‑86]], [[Interstate 87 (disambiguation)|I-87]], and [[Interstate 88 (disambiguation)|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 [[Interstate 84 (west)|I‑84]] was I‑80N, as it went north from [[Interstate 80|I‑80]]. The new policy stated, "No new divided numbers (such as [[Interstate 35W (disambiguation)|I-35W]] and [[Interstate 35E (disambiguation)|I-35E]], etc.) shall be adopted." The new policy also recommended that existing divided numbers be eliminated as quickly as possible; however, an [[Interstate 35W (Texas)|I-35W]] and [[Interstate 35E (Texas)|I-35E]] still exist in the [[Dallas–Fort Worth metroplex]] in Texas, and an [[Interstate 35W (Minnesota)|I-35W]] and [[Interstate 35E (Minnesota)|I-35E]] that run through [[Minneapolis]] and [[Saint Paul, Minnesota|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 [[Interstate 69W|I-69W]], [[Interstate 69E|I-69E]], and [[Interstate 69C|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 (road)|concurrency]] or overlap. For example, [[Interstate 75 in Georgia|I‑75]] and [[Interstate 85 in Georgia|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, Virginia|Wytheville]] and [[Fort Chiswell, Virginia|Fort Chiswell]], Virginia, where [[Interstate 81 in Virginia|I‑81]] north and [[Interstate 77 in Virginia|I‑77]] south are equivalent (with that section of road traveling almost due east), as are I‑81 south and I‑77 north.