List of auxiliary Interstate Highways

The auxiliary Interstate Highways (also called 3-digit Interstate Highways) is an additional network of freeways of the Interstate Highway System. Similar to the primary Interstate Highways, they also meet Interstate Standards, receive the same substantial federal funding, and comply with other federal standards. The main purpose of the auxiliary Interstate Highways is to serve large cities and suburbs and provide a supplement to the main Interstate Highways throughout the United States.

In contrast to the primary Interstate Highways, the three-digit highways often run only within or around a single metropolitan city. In addition, they are generally shorter, and many do not cross state borders, despite the term "Interstate". Due to the large number of these routes, auxiliary route numbers may be repeated in different states along the mainline. However, no two three-digit Interstates in the same state can share the same number.

Terminology and guidelines

The basic tenets of the auxiliary Interstates are divided into three branches: spur, loop, and bypass routes. Each one signifies a different characteristic of the auxiliary route.

The first digit of the three digits 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 indicates that this freeway is a spur. The last two digits signify the highway's origin. In this case, the "15" in I-515 supplements to I-15.

pur route

A spur route may begin at a large highway and terminate at a regular city street in a downtown area of a metropolitan city. A spur route is preceded by an odd number in the first digit.cite web|url=http://www.fhwa.dot.gov/reports/routefinder/index.cfm#s09|title=FHWA Route Log and Finder List: Interstate Route Numbering|accessdate=2008-02-03|date=2002-10-31|format=CFM|publisher=Federal Highway Administration]

*Examples of spur routes are listed below. : I-110 in California travels into downtown Los Angeles.: I-180 in Nebraska makes its way through Lincoln.

Sometimes, a three-digit highway branches off another three-digit highway; this is called a tertiary spur route of a two-digit highway. Tertiary spurs do not meet their parent highways, but are associated with them via the three-digit highways they do meet.
*Examples of tertiary spur routes are listed below. :I-370 in Maryland branches off I-270, and is thus a tertiary spur of I-70.:I-380 in California is located at the San Francisco Bay Area; the route begins at I-280 and never connects to its parent I-80.

Bypass

A bypass route may traverse around a city. 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.x]

*Examples of bypass routes are listed below. :I-215 in Utah travels the perimeter of Salt Lake City.:I-895 in Maryland serves as a bypass of downtown Baltimore.

Beltway

A beltway (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. Beltways are also preceded by an even number in the first digit.
*Examples of beltways are listed below. :I-465 in Indiana travels around the city of Indianapolis.:I-285 in Georgia engulfs the city of Atlanta.

Exceptions

*A contiguous loop surrounds the entire Minneapolis-St. Paul Metro area. I-94 intersects the loop in two spots and runs directly through it separating it into southern and northern halves; the southern half of it is labeled I-494 while the northern half of it is labeled I-694. The northern half rejoins I-94 approximately five miles before the southern half does; this stretch is signed I-94/I-694.
*I-270 and I-255 form a beltway around the greater St. Louis area. On its southwest corner, I-270 becomes I-255 as it crosses I-55 counter-clockwise only to terminate back at I-270 in the northeast. In the early 1980s, local residents stopped a plan to designate the entire closed loop as I-270 and renumber the stub of I-270 from Glen Carbon to Edwardsville to Interstate 870.Fact|date=September 2007
*New York City has numerous spur routes from I-78 and I-95, but none of I-78's spur routes actually intersects with I-78. This is because I-78 was originally planned to continue east from its present terminus at the Holland Tunnel across Lower Manhattan, through Queens and across Long Island. This extension of I-78 was cancelled in the 1970s after I-278, I-478, I-678, and I-878 had already been established.
*In the New York City area, I-495 does not connect to I-95. The connector from the New Jersey Turnpike to the Lincoln Tunnel was to have been part of I-495 despite not currently being up to interstate standards, but became NJ 495 due to the cross-Manhattan part not being built. The cross-Manhattan part was to have connected the Lincoln Tunnel to the Queens-Midtown Tunnel. I-495 currently runs from the Queens-Midtown Tunnel to a terminus near Riverhead, Suffolk County, Long Island. An extension from there, crossing Long Island Sound to I-95 in Connecticut, was never built.
*An auxiliary route numbered I-238 connects San Leandro and Castro Valley, California, yet there is no I-38. Instead, I-238 carries the designation of the portion of State Route 238 that the Interstate designation replaced. I-238 could have been an auxiliary route of I-80, but at the time of I-238's designation, all child routes of I-80 (I-280 through I-980; I-180 was not used because of a Caltrans policy forbidding number duplications and the prior existence of State Route 180) were already designated in California.

Auxiliary Interstates

Below is a list of auxiliary Interstate Highway designations in the United States. They are sorted in the following order:
# numerically by the primary Interstate Highway (one or two digits)
# alphabetically by the state of the auxiliary Interstate
# numerically by the auxiliary Interstate itself

ee also

* List of proposed auxiliary Interstate Highways
* List of unsigned auxiliary Interstate Highways

References

External links

* [http://www.kurumi.com/roads/3di/index.html 3-digit Interstates at Kurumi.com]