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Auxiliary Interstate Highways (also called three-digit Interstate Highways) are a supplemental subset of the
freeway A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms ...
s within the
Interstate Highway System The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly known as the Interstate Highway System, is a network of controlled-access highways that forms part of the National Highway System in the United States. T ...
of the United States. Auxiliary routes are generally classified as
spur route A spur route is a short road forming a branch from a longer, more important road such as a freeway, Interstate Highway, or motorway. A bypass or beltway should not be considered a true spur route as it typically reconnects with another or the ...
s, which connect to the parent route at one end, bypasses, which connect to the parent route at both ends, or beltways, which form a complete circle intersecting the parent route at two locations. There are 323 auxiliary Interstates in the United States. There are some routes which connect to the parent route at one end, but connect to another route at the other end; some states treat these as spurs while others treat them as bypasses. Similar to the mainline Interstate Highways, these highways also meet all
Interstate Highway standards Standards for Interstate Highways in the United States are defined by the American Association of State Highway and Transportation Officials (AASHTO) in the publication ''A Policy on Design Standards: Interstate System''. For a certain highway to ...
(with rare exceptions), and they receive the same percentage of federal funding (90%). The shorter auxiliary routes branch off main routes and are numbered based on the number of the parent route. All of the supplement routes for
Interstate 95 Interstate 95 (I-95) is the main north–south Interstate Highway on the East Coast of the United States, running from US Route 1 (US 1) in Miami, Florida, to the Houlton–Woodstock Border Crossing between Maine and the Canadia ...
(I-95) are designated with a three-digit number ending in "95" in the form I-x95. While some exceptions do exist, generally spur routes are numbered with an odd hundreds digit (such as
I-395 Interstate 395 may refer to: * Interstate 395 (Connecticut–Massachusetts), a spur from I-95 to Auburn, Massachusetts *Interstate 395 (Delaware), a proposed portion of I-95 in Delaware, when it was under construction *Interstate 395 (Florida), a sp ...
), while bypasses and beltways are numbered with an even hundreds digit (such as I-695). Because longer Interstates may have many such supplemental routes, the numbers can repeat from state to state along their route, but they will not repeat within a state. There are only three states that do not have any auxiliary Interstate Highways: Alaska, Arizona, and New Mexico. North Dakota has an auxiliary route, but it is unsigned, and Wyoming's does not meet Interstate Highway standards.


Terminology and guidelines

The basic tenets of the auxiliary Interstates are divided into three branches:
spur A spur is a metal tool designed to be worn in pairs on the heels of riding boots for the purpose of directing a horse or other animal to move forward or laterally while riding. It is usually used to refine the riding aids (commands) and to ba ...
,
loop Loop or LOOP may refer to: Brands and enterprises * Loop (mobile), a Bulgarian virtual network operator and co-founder of Loop Live * Loop, clothing, a company founded by Carlos Vasquez in the 1990s and worn by Digable Planets * Loop Mobile, an ...
, and bypass routes. Each one signifies a different characteristic of the auxiliary route. The first digit of the three digits usually determines whether a route is a bypass, spur, or beltway. The last two digits are derived from the main Interstate Highway. For instance, I-515 contains an odd number in the first digit (5), and this indicates that this freeway is a spur. The last two digits signify the highway's origin. In this case, the "15" in I-515 shows that it is a supplement to
I-15 I15 may refer to: * Interstate 15, a north–south Interstate Highway in the United States of America * Polikarpov I-15, a Soviet fighter aircraft * I15 (band) "Soulja Girl" is the second single from American rapper Soulja Boy's studio album '' ...
. Numerous exceptions to the standard numbering guidelines exist in many places. This can be for a number of reasons. In some cases, original routes were changed, extended, or abandoned, leaving discrepancies in the system. In other cases, it may not be possible to use the proper number because the limited set of available numbers has been exhausted, causing a "non-standard" number to be used.


Spur route

A
spur route A spur route is a short road forming a branch from a longer, more important road such as a freeway, Interstate Highway, or motorway. A bypass or beltway should not be considered a true spur route as it typically reconnects with another or the ...
's number usually has an odd number for its first digit. It is usually one of the following: * It may serve another section of a city or metropolitan area not served by the main freeway (most often the
central business district A central business district (CBD) is the commercial and business centre of a city. It contains commercial space and offices, and in larger cities will often be described as a financial district. Geographically, it often coincides with the "city ...
), terminating at a regular city street/avenue or at a substandard freeway, such as I-185 in
Columbus, Georgia Columbus is a consolidated city-county located on the west-central border of the U.S. state of Georgia. Columbus lies on the Chattahoochee River directly across from Phenix City, Alabama. It is the county seat of Muscogee County, with which it o ...
. * It may represent the first portion of a contemplated extended freeway, one that downgrades to below Interstate standards with plans to upgrade it later on. An example is I-540 in
Arkansas Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the O ...
. * It may connect two unrelated Interstate highways—as I-390 in
New York State New York, officially the State of New York, is a state in the Northeastern United States. It is often called New York State to distinguish it from its largest city, New York City. With a total area of , New York is the 27th-largest U.S. sta ...
and
I-355 Interstate 355 (I-355), also known as the Veterans Memorial Tollway, is an Interstate Highway and tollway in the western and southwest suburbs of Chicago in the U.S. state of Illinois. Like most other toll roads in the northeastern portion of th ...
in
Illinois Illinois ( ) is a state in the Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolitan areas include, Peoria and Rock ...
do. ** Note that states differ on their interpretation of the numbering convention in this case. In the I-390 example above, the route has both ends at Interstates, but not at the same Interstate on both ends, and is assigned an odd first digit. Another example is I-275 in Tennessee: it is a connector between
I-40 Interstate 40 (I-40) is a major east–west Interstate Highway running through the south-central portion of the United States. At a length of , it is the third-longest Interstate Highway in the country, after I-90 and I-80. From west to ea ...
and
I-75 Interstate 75 (I-75) is a major north–south Interstate Highway in the Great Lakes and Southeastern regions of the United States. As with most Interstates that end in 5, it is a major cross-country, north–south route, traveling from St ...
(a similar case of having both ends at Interstates but not at the same Interstate) and is assigned an even first digit. Examples include: * I-110 in
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
links
I-10 Interstate 10 (I-10) is the southernmost cross-country highway in the American Interstate Highway System. I-10 is the fourth-longest Interstate in the United States at , following I-90, I-80, and I-40. This freeway is part of the originally p ...
with the
Port of Los Angeles The Port of Los Angeles is a seaport managed by the Los Angeles Harbor Department, a unit of the City of Los Angeles. It occupies of land and water with of waterfront and adjoins the separate Port of Long Beach. Promoted as "America's Port", ...
. This freeway was built before Interstate Highways as the Harbor Freeway. Later, it was adopted into the Interstate Highway System. * I-180 in North-Central Pennsylvania connects Williamsport and
Lycoming County Lycoming County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 114,188. Its county seat A county seat is an administrative center, seat of government, or capital city of a county or civil parish. ...
with
I-80 Interstate 80 (I-80) is an east–west transcontinental freeway that crosses the United States from downtown San Francisco, California, to Teaneck, New Jersey, in the New York metropolitan area. The highway was designated in 1956 as one o ...
in Milton, which does not actually enter Lycoming County. I-180 also serves as a connection to those traveling to
New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...
and
I-86 Interstate 86 may refer to any of three unconnected Interstate Highways in the United States: * Interstate 86 (Pennsylvania–New York) * Interstate 86 (Idaho) * Interstate 84 (Pennsylvania–Massachusetts) Interstate 84 (I-84) is an I ...
via U.S. Route 15 and the Future I-99 Corridor at its western terminus. *
I-190 Interstate 190 may refer to the following Interstate Highways in the United States related to Interstate 90: * Interstate 190 (Illinois), a spur into Chicago's O'Hare International Airport *Interstate 190 (Massachusetts), a spur from Worcester to Le ...
in New York connects the cities of
Niagara Falls Niagara Falls () is a group of three waterfalls at the southern end of Niagara Gorge, spanning the Canada–United States border, border between the Provinces and territories of Canada, province of Ontario in Canada and the U.S. state, state ...
and Buffalo with
I-90 Interstate 90 (I-90) is an east–west transcontinental freeway and the longest Interstate Highway in the United States at . It begins in Seattle, Washington, and travels through the Pacific Northwest, Mountain West, Great Plains, Midwest, an ...
. Sometimes, a three-digit Interstate Highway branches off from another three-digit Interstate Highway. These spurs do not connect directly with their parent highways, but are associated with them via the three-digit highways they do intersect with. Examples include: *
I-380 Interstate 380 may refer to: * Interstate 380 (California) Interstate 380 (I-380) is a short east–west auxiliary Interstate Highway in the San Francisco Bay Area of Northern California, connecting I-280 in San Bruno to US Route& ...
in
Northern California Northern California (colloquially known as NorCal) is a geographic and cultural region that generally comprises the northern portion of the U.S. state of California. Spanning the state's northernmost 48 counties, its main population centers incl ...
is located at the
San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area G ...
. This highway begins at I-280, and it connects with US 101 and
San Francisco International Airport San Francisco International Airport is an international airport in an unincorporated area of San Mateo County, south of Downtown San Francisco. It has flights to points throughout North America and is a major gateway to Europe, the Middl ...
. *
I-190 Interstate 190 may refer to the following Interstate Highways in the United States related to Interstate 90: * Interstate 190 (Illinois), a spur into Chicago's O'Hare International Airport *Interstate 190 (Massachusetts), a spur from Worcester to Le ...
in
Massachusetts Massachusetts (Massachusett: ''Muhsachuweesut Massachusett_writing_systems.html" ;"title="nowiki/> məhswatʃəwiːsət.html" ;"title="Massachusett writing systems">məhswatʃəwiːsət">Massachusett writing systems">məhswatʃəwiːsət'' En ...
branches off from
I-290 Interstate 290 may refer to the following Interstate Highways in the United States: *Interstate 290 (Illinois), a highway from Chicago, Illinois, to Rolling Meadows, Illinois * Interstate 290 (Massachusetts), a highway in Massachusetts that connects ...
near
Worcester Worcester may refer to: Places United Kingdom * Worcester, England, a city and the county town of Worcestershire in England ** Worcester (UK Parliament constituency), an area represented by a Member of Parliament * Worcester Park, London, Engla ...
. * I-795 in
Baltimore County, Maryland Baltimore County ( , locally: or ) is the third-most populous county in the U.S. state of Maryland and is part of the Baltimore metropolitan area. Baltimore County (which partially surrounds, though does not include, the independent City o ...
branches off from I-695.


Bypass

A bypass route may traverse around a city, or may run through it with the mainline bypassing. In a typical 3-digit Interstate Highway, bypasses usually have both its two termini junctioned with another Interstate highway. Bypass routes are preceded by an even number in the first digit. Examples include: *
I-220 Interstate 220 (I-220) is the designation for two Interstate Highways in the United States, both related to Interstate 20: *Interstate 220 (Mississippi), a bypass of Jackson, Mississippi *Interstate 220 (Louisiana) Interstate 220 (I-220) in L ...
in
Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is bord ...
serves as a bypass of downtown
Shreveport Shreveport ( ) is a city in the U.S. state of Louisiana. It is the third most populous city in Louisiana after New Orleans and Baton Rouge, respectively. The Shreveport–Bossier City metropolitan area, with a population of 393,406 in 2020, is ...
. * I-440 forms a loop around the south side of
Nashville Nashville is the capital city of the U.S. state of Tennessee and the seat of Davidson County. With a population of 689,447 at the 2020 U.S. census, Nashville is the most populous city in the state, 21st most-populous city in the U.S., and th ...
. * I-890 travels through downtown
Schenectady Schenectady () is a city in Schenectady County, New York, United States, of which it is the county seat. As of the 2020 census, the city's population of 67,047 made it the state's ninth-largest city by population. The city is in eastern New Y ...
, which
I-90 Interstate 90 (I-90) is an east–west transcontinental freeway and the longest Interstate Highway in the United States at . It begins in Seattle, Washington, and travels through the Pacific Northwest, Mountain West, Great Plains, Midwest, an ...
avoids. In the case of an auxiliary Interstate highway which has both ends at Interstates but not the same Interstate, some states treat these as bypasses while others treat these as spurs — see ''Spur route'' above.


Beltway

A
beltway A ring road (also known as circular road, beltline, beltway, circumferential (high)way, loop, bypass or orbital) is a road or a series of connected roads encircling a town, city, or country. The most common purpose of a ring road is to assist i ...
(also known as a loop route) completely surrounds a metropolitan city, and it is often connected with multiple junctions to other routes. Unlike other auxiliary Interstate Highways, beltways do not have termini; however, they have a place where the highway mileage resets to zero. Beltways are also preceded by an even number in the first digit. Some examples of beltways include: * I-275 in
Ohio Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...
,
Kentucky Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virginia ...
and
Indiana Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th ...
encircles the city of
Cincinnati Cincinnati ( ) is a city in the U.S. state of Ohio and the county seat of Hamilton County. Settled in 1788, the city is located at the northern side of the confluence of the Licking and Ohio rivers, the latter of which marks the state line w ...
. *
I-465 Interstate 465 (I-465), also known as the USS ''Indianapolis'' Memorial Highway, is the beltway circling Indianapolis, Indiana, United States. It is roughly rectangular in shape and has a perimeter of approximately . It lies almost compl ...
in
Indiana Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th ...
encircles the city of
Indianapolis Indianapolis (), colloquially known as Indy, is the state capital and most populous city of the U.S. state of Indiana and the seat of Marion County. According to the U.S. Census Bureau, the consolidated population of Indianapolis and Marion ...
. * I-495 in
Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean t ...
and
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth are ...
encircles the city of Washington, D.C. and is referred to as "The Capital Beltway".


Auxiliary Interstates

:''Note: this table sorts the route numbers by parent highway.''


See also

*


Notes


References


External links

*
3-digit Interstates at Kurumi.com
{{DEFAULTSORT:List Of Auxiliary Interstate Highways * *Z Interstate Aux es:Anexo:Autopistas Interestatales Primarias