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. Th ...

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 ...

s, which connect to the parent route at one end,

bypasses Bypass may refer to: * Bypass (road), a road that avoids a built-up area (not to be confused with passing lane) * Flood bypass of a river Science and technology Medicine * Bypass surgery, a class of surgeries including for example: ** Heart bypa ...

, 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 U.S. Route 1, US Route 1 (US 1) in Miami, Miami, Florida, to the Houlton–Woodstock Border Crossing between M ...

(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), 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 Unsigned can refer to: * An unsigned artist is a musical artist or group not attached or signed to a record label ** Unsigned Music Awards, ceremony noting achievements of unsigned artists ** Unsigned band web, online community * Similarly, the c ...

, 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 back ...

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. 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

'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 ...

. * 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 Osage ...

. * 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. stat ...

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 ...

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...

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 Interstate 275 (I-275) may refer to: *Interstate 275 (Florida), a loop through Tampa, St. Petersburg, and Bradenton in Florida *Interstate 275 (Michigan), a western bypass of Detroit, Michigan * Interstate 275 (Ohio–Indiana–Kentucky), a full bel ...

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 Interstate 110 may refer to: *Interstate 110 and State Route 110 (California), Interstate 110 (California), a north–south freeway running through Los Angeles, California *Interstate 110 (Florida), a spur route in Pensacola, Florida *Interstate 1 ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

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 pl ...

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", t ...

. This freeway was built before Interstate Highways as the

Harbor Freeway A harbor (American English), harbour (British English; see spelling differences), or haven is a sheltered body of water where ships, boats, and barges can be docked. The term ''harbor'' is often used interchangeably with ''port'', which is a ...

. 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 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 U.S. Route 15 (US 15) is a -long United States highway, designated along South Carolina, North Carolina, Virginia, Maryland, Pennsylvania, and New York. The route is signed north–south, from U.S. Route 17 Alternate in Walterboro, South Caro ...

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 L ...

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 border between the province of Ontario in Canada and the state of New York in the United States. The largest of the three is Horseshoe Falls, ...

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, and ...

. 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& ...

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 Go ...

. This highway begins at

I-280 Interstate 280 may refer to multiple highways, all of which are or were related to Interstate 80: * Interstate 280 (California), a north–south freeway running from San Jose to San Francisco * Interstate 280 (Iowa–Illinois), part of the beltway ...

, 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 Middle E ...

. *

Massachusetts Massachusetts (Massachusett language, Massachusett: ''Muhsachuweesut assachusett writing systems, məhswatʃəwiːsət'' English: , ), officially the Commonwealth of Massachusetts, is the most populous U.S. state, state in the New England ...

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 of ...

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 ...

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 borde ...

serves as a bypass of downtown Shreveport. * 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 the ...

. *

I-890 Interstate 890 (I-890) is a auxiliary Interstate Highway in the vicinity of Schenectady, New York, in the United States. The highway runs southeast–northwest from an interchange with the New York State Thruway ( I-90) northwest of Schene ...

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

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

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 In road transportation in the United States, a special route is a road in a numbered highway system that diverts a specific segment of related traffic away from another road. They are featured in many highway systems; most are found in the Int ...

) 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: *

Ohio Ohio () is a state in the Midwestern region of the United States. Of the fifty U.S. states, it is the 34th-largest by area, and with a population of nearly 11.8 million, is the seventh-most populous and tenth-most densely populated. The sta ...

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 to ...

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 s ...

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 wit ...

. * I-465 in

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 Interstate 495 (I-495) is the designation for several Interstate Highways in the United States, all of which are related to Interstate 95, listed from south to north: * The Capital Beltway, a beltway around Washington, D.C., running through Virgin ...

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 to ...

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 ar ...

encircles the city of

Washington, D.C. ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...

and is referred to as "The Capital Beltway".

Auxiliary Interstates

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

Notes

References

External links

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

Terminology and guidelines

Spur route

Bypass

Beltway

Auxiliary Interstates

See also

Notes

References

External links