Editing Interstate Highway System (section)

==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.-->
{{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.

===Auxiliary (three-digit) Interstates===
{{See also|List of auxiliary Interstate Highways}}
[[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 [[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, [[Interstate 190 (Massachusetts)|I-190]] in Massachusetts is labeled north–south, while [[Interstate 195 (New Jersey)|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 [[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 ([[Interstate H-1|H-1]], [[Interstate H-2|H-2]], and [[Interstate H-3|H-3]]) and one auxiliary route ([[Interstate H-201|H-201]]). These Interstates connect several [[United States Air Force|military]] and [[United States Navy|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 [[Interstate Highways in Alaska|Interstates of Alaska]] and [[Interstate Highways in Puerto Rico|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 [[Interstate 495 (Capital Beltway)|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 in the United States|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, [[Interstate 19|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 [[List of business routes of the Interstate Highway System|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 [[Interstate 75 Business (Pontiac, Michigan)|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 [[U.S. Route 10 in Michigan|US&nbsp;10]] and [[M-24 (Michigan highway)|M-24]], predecessors of [[Interstate 75 in Michigan|I-75]] in the area.