ways) in the United States that is named for the president who was in office when the system was created. The Interstate Highway System is a separate system within the larger National Highway System. The entire system, as of 2004, had a total length of 46,837 miles (75,376 km) [1].While Interstate Highways usually receive substantial federal funding and comply with federal standards, they are owned, built, and operated by the states or toll authorities. The original Woodrow Wilson Bridge, part of Interstate 95 and Interstate 495, was maintained by the federal government; its new span is now jointly owned and maintained by the State of Maryland and the Commonwealth of Virginia .[2]
The system serves nearly all major U.S. cities. Unlike counterparts in most other industrialized countries[citation needed], many Interstates pass through downtown areas. This has helped to facilitate the emergence of automobile-oriented postwar suburban development patterns, a phenomenon also known as urban sprawl.[citation needed]
The system is prominent in American daily life. The distribution of virtually all goods and services involves Interstate Highways at some point.[citation needed] Residents of American cities commonly use urban Interstates to travel to their places of work. The vast majority of long-distance travel, whether for vacation or business, uses the national road network;[3] of these trips, about one-third (by the total number of miles driven in the country in 2003) utilize the Interstate system.[4]
History
The Interstate System was authorized by the Federal-Aid Highway Act of 1956, popularly known as the National Interstate and Defense Highways Act of 1956. It had been lobbied for by major U.S. automobile manufacturers and championed by President Dwight D. Eisenhower- who was influenced by both his experiences as a young soldier crossing the country i
n 1919 following the route of the Lincoln Highway and his appreciation of the German autobahn network - as a necessary component of a national defense system. [5] It would be able to provide key ground transport of military supplies and troop deployments.
Initial federal planning for a nationwide highway system began in 1921 when the Bureau of Public Roads asked the Army to provide it with a list of roads it considered necessary for national defense, resulting in the Pershing Map.[6] Later that decade, highways such as the New York parkway system had been built as part of local or state highway systems. As automotive traffic increased, planners saw a need for such an interconnected national system to supplement the existing, largely non-freeway, United States Numbered Highway system. By the late 1930s, planning had expanded to a system of new superhighways. In 1938, President Franklin D. Roosevelt gave BPR chief Thomas MacDonald a hand-drawn map of the U.S. marked with eight superhighway corridors for study.[6] The publication General location of national system of interstate highways maps out what became the Interstate System, and is informally known as the Yellow Book. [1].
Although construction on the Interstate Highway system continues, the removal of the last traffic signal on Interstate 90 in Wallace, Idaho, on September 15, 1991 is often cited as the completion of the Interstate System. The initial cost estimate for the system was $25 billion over twelve years; it ended up costing $114 billion (adjusted for inflation, $425 billion in 2006 dollars[7] and taking 35 years to complete [2].
However, due to the cancellation of the Somerset Freeway, Interstate 95 is discontinuous in New Jersey. When the Pennsylvania Turnpike/Interstate 95 Interchange Project concludes around 2009 , the last section of the basic original plan will be completed.
Missouri and Kansas have laid claims that I-70 in those states was the first interstate (Missouri also says I-44 is one of the first). The first three contracts under the new program were signed in Missouri on August 2, 1956. Kansas claims that it was the first to start paving after the act and that the Kansas portion of I-70 was the first complete section. However, construction and planning on those roads had been underway before the highway act. The Pennsylvania Turnpike, which opened on October 1, 1940, claims to be the first limited-access, divided highway in the country.[8]
Standards
Main article: Interstate Highway standards
The American Association of State Highway and Transportation Officials (AASHTO) has defined a set of standards that all new Interstates must meet unless a waiver from the Federal Highway Administration (FHWA) is obtained. These standards have become more strict over the years.[citation needed] One almost absolute standard is the controlled access nature of the roads. With few exceptions, traffic lights (and cross traffic in general) are limited to toll booths and ramp meters (metered flow control for lane merging during rush hour).
Speed limits
Further information: Speed limits in the United States
Interstate Highways often have the highest speed limits in a given area. Speed limits are determined by individual states.[citation needed] Rural limits generally range from 65 to 80 mph (100 to 130 km/h) typically with the lower limits in the more populated northeastern states and the higher speeds in the western states[citation needed], with the exception of Oregon's state speed limit ceiling of 65 mph.[citation needed] Urban interstate speed limits are generally 50 to 65 mph (80.5 to 104.6 km/h) across the country.[citation needed]
Dual-purpose design
Further information: Contraflow lane reversal
In addition to being designed to support automobile and heavy truck traffic, interstate highways are also designed for use in military and civil defense operations within the United States, particularly troop movements.[citation needed]
One potential civil defense use of the Interstate Highway System is for the emergency evacuation of cities in the event of a potential nuclear war.[citation needed] The Interstate Highway System has been used to facilitate evacuations in the face of h
urricanes and other natural disasters.
An option for maximizing traffic throughput on a highway is to reverse the flow of traffic on one side of a divider so that all lanes become outbound lanes. This procedure, known as contraflow, was first employed in the 1998 evacuation of New Orleans, Louisiana, in preparation for Hurricane Georges.[citation needed] In 2004, contraflow was employed ahead of Hurricane Charley in the Tampa, Florida area and on the Gulf Coast before the landfall of Hurricane Ivan.[9] It was again used in 2005 during the evacuations of New Orleans, Louisiana, and Houston, Texas, prior to hurricanes Katrina and Rita, respectively.[citation needed]
A widespread urban legend states that one out of every five miles of the Interstate Highway System must be built straight and flat so as to be usable by aircraft during times of war. Contrary to popular lore, Interstate highways are not designed to serve as airstrips. [10][11]
Terminology
While the name implies that Interstate highways cross state lines, many do not (for details see List of intrastate Interstates). Rather, they are funded federally with money shared among the states. There are interstate highways in Hawaii, funded in the same way as in the other states, but entirely within the populous island of Oahu. They have the designation of H-x and connect military bases. Similarly, both Alaska and Puerto Rico have public roads that receive funding from the Interstate program, though these routes are not signed as Interstate Highways except "on paper". These roads are neither planned for nor currently built to Interstate Highway standards.
The numbering system can be summarized in these general rules. One- and two-digit highways are generally national routes:
Odd numbered routes run north-south, and higher numbers are farther east.[12]
I-5 runs up and down the West Coast, with I-95 on the East Coast.
Even numbered routes run east-west, and higher numbers are farther north.[12]
I-10 runs along the Southern states, and I-90 runs near the US-Canada border.
Three-digit highways, or Auxiliary Interstate Highways, run only within a single city.
An odd first digit signifies a spur route, which may begin at a large highway and terminate at a city center.[12]
I-110 travels into downtown Los Angeles.
An even first digit signifies a loop route around a city or a bypass through a city.[12]
I-215 travels the perimeter of Salt Lake City.
I-895 serves as a bypass of downtown Baltimore.
The last two digits signify the highway's origin.[12]
I-480 eventually returns to I-80, a primary interstate highway.
I-185 in Georgia leaves I-85 to go into Columbus.
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.[citation needed]
I-370 in Maryland branches off I-270, and is thus a tertiary spur of I-70.
I-380 in the San Francisco Bay Area begins at I-280 and never connects to its parent I-80.
Primary routes
See also: List of Interstate Highways
Interstate highways are typically known as Interstate XX or I-XX, where "XX" is the one- or two-digit route number; sometimes Interstate Highway XX (IH XX) or Interstate Route XX (IR XX) is used. In some areas, the more generic Route XX or Highway XX is used, or in the case of Southern California, Nevada, and New York, The XX.
The numbering scheme for the Interstate Highway System (as well as the U.S. Highway System
) was developed in 1957 by the American Association of State Highway and Transportation Officials (AASHTO)[12] and is still coordinated by them, though their authority is occasionally trumped by a number written into Federal law. Within the continental United States, primary Interstates (also called main line Interstates or two-digit Interstates) are given one- or two-digit route numbers. Most Interstates have two digits; there are only three one-digit Interstates in the system: I-4, I-5 and I-8. Within this category, 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, though there are exceptions to both principles in several locations. Numbers divisible by 5 are intended to be major among the primary routes, carrying traffic long distances.[citation needed] For example, I-5 runs from Canada to Mexico along the west coast (the only Interstate to do so) while I-95 runs from Miami north to Canada along the east coast. In addition, I-10 runs from Santa Monica, California, to Jacksonville, Florida, while I-90 runs from Seattle, to Boston. However, not all primary routes divisible by 5 traverse long distances. I-45 runs from Galveston, Texas, north to Dallas, a distance of only 284 miles (457 km). It is the only primary route divisible by 5 that does not cross state lines. See List of intrastate Interstate Highways for other primary routes that do not cross state lines.
I-50 and I-60 do not exist (and there are no even-numbered Interstates from 46 to 62), mainly because they would most likely have passed through the same states that already have US 50 and US 60. AASHTO rules discourage Interstate and U.S. Highways having the same number within the same state, although I-24 and US 24 exist at opposite ends of Illinois. Some planned Interstates do not follow this guideline — I-69 will intersect US 69 in Lufkin, Texas[13], I-74 will overlap US 74 in North Carolina, and I-41 will do the same with US 41 in Wisconsin.
Several two-digit numbers are shared between two roads at opposite ends of the country, namely I-76, I-84, I-86 and I-88. Some of these were the result of a change in the numbering system in the 1970s; previously letter-suffixed numbers were used for long spurs off primary routes; for example, western I-84 was I-80N, as it went north from I-80. In the 1970s, AASHTO decided to eliminate these[citation needed]; some became additional two-digit routes, while others became three-digit routes (see below). Only two pairs of these still exist; I-35 splits into I-35W and I-35E through both the Dallas-Fort Worth and the Minneapolis-St. Paul areas.
For the sake of efficiency, some Interstates double up for short or sometimes long distances. This is usually referred to as a concurrency or overlap. One example is where I-75 and I-85 combine near downtown Atlanta to form the Downtown Connector, a major thoroughfare through the city. Another example is the merging and diverging of Interstates 90 and 94, which double and then separate several times across the upper Midwest and Great Plains. I-90 and I-94 even join with I-39 from Madison to Portage, Wisconsin, creating the longest triple concurrence in the Interstate system. Interstates 90 and 80 are concurrent for almost 280 miles in Indiana and Ohio. A recent addition is the I-73/I-74 concurrency, which runs along U.S. Route 220 from Asheboro to Rockingham, North Carolina. I-73 will also share routes with I-40 and I-85 on the Greensboro Urban Loop, a new bypass route around Greensboro, North Carolina. Strict adherence to the directional nature of the system results in some oddities. For a nine-mile (14 km) stretch east of Wytheville, Virginia, the driver can be traveling on both I-81 North and I-77 South at the same time (and vice versa) (see also wrong-way concurrency).
Auxiliary Interstates
n 1919 following the route of the Lincoln Highway and his appreciation of the German autobahn network - as a necessary component of a national defense system. [5] It would be able to provide key ground transport of military supplies and troop deployments.Initial federal planning for a nationwide highway system began in 1921 when the Bureau of Public Roads asked the Army to provide it with a list of roads it considered necessary for national defense, resulting in the Pershing Map.[6] Later that decade, highways such as the New York parkway system had been built as part of local or state highway systems. As automotive traffic increased, planners saw a need for such an interconnected national system to supplement the existing, largely non-freeway, United States Numbered Highway system. By the late 1930s, planning had expanded to a system of new superhighways. In 1938, President Franklin D. Roosevelt gave BPR chief Thomas MacDonald a hand-drawn map of the U.S. marked with eight superhighway corridors for study.[6] The publication General location of national system of interstate highways maps out what became the Interstate System, and is informally known as the Yellow Book. [1].
Although construction on the Interstate Highway system continues, the removal of the last traffic signal on Interstate 90 in Wallace, Idaho, on September 15, 1991 is often cited as the completion of the Interstate System. The initial cost estimate for the system was $25 billion over twelve years; it ended up costing $114 billion (adjusted for inflation, $425 billion in 2006 dollars[7] and taking 35 years to complete [2].
However, due to the cancellation of the Somerset Freeway, Interstate 95 is discontinuous in New Jersey. When the Pennsylvania Turnpike/Interstate 95 Interchange Project concludes around 2009 , the last section of the basic original plan will be completed.
Missouri and Kansas have laid claims that I-70 in those states was the first interstate (Missouri also says I-44 is one of the first). The first three contracts under the new program were signed in Missouri on August 2, 1956. Kansas claims that it was the first to start paving after the act and that the Kansas portion of I-70 was the first complete section. However, construction and planning on those roads had been underway before the highway act. The Pennsylvania Turnpike, which opened on October 1, 1940, claims to be the first limited-access, divided highway in the country.[8]
Standards
Main article: Interstate Highway standards
The American Association of State Highway and Transportation Officials (AASHTO) has defined a set of standards that all new Interstates must meet unless a waiver from the Federal Highway Administration (FHWA) is obtained. These standards have become more strict over the years.[citation needed] One almost absolute standard is the controlled access nature of the roads. With few exceptions, traffic lights (and cross traffic in general) are limited to toll booths and ramp meters (metered flow control for lane merging during rush hour).
Speed limits
Further information: Speed limits in the United States
Interstate Highways often have the highest speed limits in a given area. Speed limits are determined by individual states.[citation needed] Rural limits generally range from 65 to 80 mph (100 to 130 km/h) typically with the lower limits in the more populated northeastern states and the higher speeds in the western states[citation needed], with the exception of Oregon's state speed limit ceiling of 65 mph.[citation needed] Urban interstate speed limits are generally 50 to 65 mph (80.5 to 104.6 km/h) across the country.[citation needed]
Dual-purpose design
Further information: Contraflow lane reversal
In addition to being designed to support automobile and heavy truck traffic, interstate highways are also designed for use in military and civil defense operations within the United States, particularly troop movements.[citation needed]
One potential civil defense use of the Interstate Highway System is for the emergency evacuation of cities in the event of a potential nuclear war.[citation needed] The Interstate Highway System has been used to facilitate evacuations in the face of h
urricanes and other natural disasters.An option for maximizing traffic throughput on a highway is to reverse the flow of traffic on one side of a divider so that all lanes become outbound lanes. This procedure, known as contraflow, was first employed in the 1998 evacuation of New Orleans, Louisiana, in preparation for Hurricane Georges.[citation needed] In 2004, contraflow was employed ahead of Hurricane Charley in the Tampa, Florida area and on the Gulf Coast before the landfall of Hurricane Ivan.[9] It was again used in 2005 during the evacuations of New Orleans, Louisiana, and Houston, Texas, prior to hurricanes Katrina and Rita, respectively.[citation needed]
A widespread urban legend states that one out of every five miles of the Interstate Highway System must be built straight and flat so as to be usable by aircraft during times of war. Contrary to popular lore, Interstate highways are not designed to serve as airstrips. [10][11]
Terminology
While the name implies that Interstate highways cross state lines, many do not (for details see List of intrastate Interstates). Rather, they are funded federally with money shared among the states. There are interstate highways in Hawaii, funded in the same way as in the other states, but entirely within the populous island of Oahu. They have the designation of H-x and connect military bases. Similarly, both Alaska and Puerto Rico have public roads that receive funding from the Interstate program, though these routes are not signed as Interstate Highways except "on paper". These roads are neither planned for nor currently built to Interstate Highway standards.
The numbering system can be summarized in these general rules. One- and two-digit highways are generally national routes:
Odd numbered routes run north-south, and higher numbers are farther east.[12]
I-5 runs up and down the West Coast, with I-95 on the East Coast.
Even numbered routes run east-west, and higher numbers are farther north.[12]
I-10 runs along the Southern states, and I-90 runs near the US-Canada border.
Three-digit highways, or Auxiliary Interstate Highways, run only within a single city.
An odd first digit signifies a spur route, which may begin at a large highway and terminate at a city center.[12]
I-110 travels into downtown Los Angeles.
An even first digit signifies a loop route around a city or a bypass through a city.[12]
I-215 travels the perimeter of Salt Lake City.
I-895 serves as a bypass of downtown Baltimore.
The last two digits signify the highway's origin.[12]
I-480 eventually returns to I-80, a primary interstate highway.
I-185 in Georgia leaves I-85 to go into Columbus.
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.[citation needed]
I-370 in Maryland branches off I-270, and is thus a tertiary spur of I-70.
I-380 in the San Francisco Bay Area begins at I-280 and never connects to its parent I-80.
Primary routes
See also: List of Interstate Highways
Interstate highways are typically known as Interstate XX or I-XX, where "XX" is the one- or two-digit route number; sometimes Interstate Highway XX (IH XX) or Interstate Route XX (IR XX) is used. In some areas, the more generic Route XX or Highway XX is used, or in the case of Southern California, Nevada, and New York, The XX.
The numbering scheme for the Interstate Highway System (as well as the U.S. Highway System
) was developed in 1957 by the American Association of State Highway and Transportation Officials (AASHTO)[12] and is still coordinated by them, though their authority is occasionally trumped by a number written into Federal law. Within the continental United States, primary Interstates (also called main line Interstates or two-digit Interstates) are given one- or two-digit route numbers. Most Interstates have two digits; there are only three one-digit Interstates in the system: I-4, I-5 and I-8. Within this category, 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, though there are exceptions to both principles in several locations. Numbers divisible by 5 are intended to be major among the primary routes, carrying traffic long distances.[citation needed] For example, I-5 runs from Canada to Mexico along the west coast (the only Interstate to do so) while I-95 runs from Miami north to Canada along the east coast. In addition, I-10 runs from Santa Monica, California, to Jacksonville, Florida, while I-90 runs from Seattle, to Boston. However, not all primary routes divisible by 5 traverse long distances. I-45 runs from Galveston, Texas, north to Dallas, a distance of only 284 miles (457 km). It is the only primary route divisible by 5 that does not cross state lines. See List of intrastate Interstate Highways for other primary routes that do not cross state lines.I-50 and I-60 do not exist (and there are no even-numbered Interstates from 46 to 62), mainly because they would most likely have passed through the same states that already have US 50 and US 60. AASHTO rules discourage Interstate and U.S. Highways having the same number within the same state, although I-24 and US 24 exist at opposite ends of Illinois. Some planned Interstates do not follow this guideline — I-69 will intersect US 69 in Lufkin, Texas[13], I-74 will overlap US 74 in North Carolina, and I-41 will do the same with US 41 in Wisconsin.
Several two-digit numbers are shared between two roads at opposite ends of the country, namely I-76, I-84, I-86 and I-88. Some of these were the result of a change in the numbering system in the 1970s; previously letter-suffixed numbers were used for long spurs off primary routes; for example, western I-84 was I-80N, as it went north from I-80. In the 1970s, AASHTO decided to eliminate these[citation needed]; some became additional two-digit routes, while others became three-digit routes (see below). Only two pairs of these still exist; I-35 splits into I-35W and I-35E through both the Dallas-Fort Worth and the Minneapolis-St. Paul areas.
For the sake of efficiency, some Interstates double up for short or sometimes long distances. This is usually referred to as a concurrency or overlap. One example is where I-75 and I-85 combine near downtown Atlanta to form the Downtown Connector, a major thoroughfare through the city. Another example is the merging and diverging of Interstates 90 and 94, which double and then separate several times across the upper Midwest and Great Plains. I-90 and I-94 even join with I-39 from Madison to Portage, Wisconsin, creating the longest triple concurrence in the Interstate system. Interstates 90 and 80 are concurrent for almost 280 miles in Indiana and Ohio. A recent addition is the I-73/I-74 concurrency, which runs along U.S. Route 220 from Asheboro to Rockingham, North Carolina. I-73 will also share routes with I-40 and I-85 on the Greensboro Urban Loop, a new bypass route around Greensboro, North Carolina. Strict adherence to the directional nature of the system results in some oddities. For a nine-mile (14 km) stretch east of Wytheville, Virginia, the driver can be traveling on both I-81 North and I-77 South at the same time (and vice versa) (see also wrong-way concurrency).
Auxiliary Interstates
Auxiliary Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of a primary Interstate highway, to designate spur or loop routes branching from either the primary route or one of its other auxiliary routes. A spur route is one that deviates from its parent and does not end at another Interstate; it is given an odd first digit. A loop route is one that returns to its parent; it is given an even first digit. The number given to the first digit of a route that branches from the parent to end at another Interstate depends on the state; some consider these routes spurs and assign odd first digits, while others consider them loop connectors giving them even first digits.
When letter-suffixed two-digit Interstates (see above) were abundant, their auxiliary routes were given a number without a letter suffix (with the exception of I-180N in Boise, Idaho, which is now Interstate 184).
Due to the large number of these routes, auxiliary route (a.k.a. "spur route") numbers may be repeated in different states along the mainline; but no two three-digit Interstates in the same state can share a number. For instance, I-90 in New York alone has a full set of three-digit Interstates - I-190, I-290, I-390, I-490, I-590, I-690, I-790, I-890 and I-990.
Closed loops usually retain a single designation for the entire route, even when they enter other states. For example, Cincinnati, like many other cities, features a large loop around the city that intersects with the primary routes I-71, I-74, and I-75 and travels through Ohio, Kentucky, and Indiana. The entire 84 mile (135 km) loop is labeled I-275. (It is also the only three-digit interstate to ever go through three states. With the possible exception of I-495 which briefly enters the non-state Washington, DC.)
The loop highway around Washington, D.C., known as the Capital Beltway, carries both I-95 and I-495. Originally, the loop carried only I-495, in anticipation of I-95 being routed through Washington, but in 1977[citation needed], I-95 was rerouted on the eastern half of the loop due to the cancellation of the segment that would have connected downtown Washington to College Park, Maryland, while I-495 remained on the western half. I-495 was added back to the eastern half of the loop in 1989[citation needed], creating a rare concurrency of primary and auxiliary routes.
Unlike primary Interstates, three-digit Interstates are signed either east-west or north-south depending on the general orientation of the route, without any regard to the route number. For some looped Interstate routes, Inner and Outer banners are used as a directional labeling system as opposed to compass directions.
Of course, there are exceptions to these guidelines:
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.[citation needed]
New York City has numerous spur routes off from I-78 and I-95, but none of I-78's spur routes actually intersect with I-78.
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's parent route is I-80[citation needed], and 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 Caltrans policy forbidding number duplications and the prior existance of California State Route 180) were already designated in California.
Exceptions
A major exception to the overall numbering system is Interstate 99 in Pennsylvania, which was written into law as I-99 by Pennsylvania Congressman Bud Shuster; I-99 (which is also U.S. Route 220) is west of several Interstates that are numerically less than 99, but 99 was the nearest odd 2-digit number available for the interstate.
A less-notable exception is I-82, which lies fully north of I-84; this is a relic from I-84 previously having the designation of I-80N.
Some Proposed Interstate Highways have been given similarly non-conforming designations by their legislative proponents. For example, backers of the proposed Third Infantry Division Highway, a route in Georgia and Tennessee, have suggested it be named Interstate 3, in honor of the U.S. 3rd Infantry Division.[citation needed]
When letter-suffixed two-digit Interstates (see above) were abundant, their auxiliary routes were given a number without a letter suffix (with the exception of I-180N in Boise, Idaho, which is now Interstate 184).
Due to the large number of these routes, auxiliary route (a.k.a. "spur route") numbers may be repeated in different states along the mainline; but no two three-digit Interstates in the same state can share a number. For instance, I-90 in New York alone has a full set of three-digit Interstates - I-190, I-290, I-390, I-490, I-590, I-690, I-790, I-890 and I-990.
Closed loops usually retain a single designation for the entire route, even when they enter other states. For example, Cincinnati, like many other cities, features a large loop around the city that intersects with the primary routes I-71, I-74, and I-75 and travels through Ohio, Kentucky, and Indiana. The entire 84 mile (135 km) loop is labeled I-275. (It is also the only three-digit interstate to ever go through three states. With the possible exception of I-495 which briefly enters the non-state Washington, DC.)
The loop highway around Washington, D.C., known as the Capital Beltway, carries both I-95 and I-495. Originally, the loop carried only I-495, in anticipation of I-95 being routed through Washington, but in 1977[citation needed], I-95 was rerouted on the eastern half of the loop due to the cancellation of the segment that would have connected downtown Washington to College Park, Maryland, while I-495 remained on the western half. I-495 was added back to the eastern half of the loop in 1989[citation needed], creating a rare concurrency of primary and auxiliary routes.
Unlike primary Interstates, three-digit Interstates are signed either east-west or north-south depending on the general orientation of the route, without any regard to the route number. For some looped Interstate routes, Inner and Outer banners are used as a directional labeling system as opposed to compass directions.
Of course, there are exceptions to these guidelines:
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.[citation needed]
New York City has numerous spur routes off from I-78 and I-95, but none of I-78's spur routes actually intersect with I-78.
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's parent route is I-80[citation needed], and 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 Caltrans policy forbidding number duplications and the prior existance of California State Route 180) were already designated in California.
Exceptions
A major exception to the overall numbering system is Interstate 99 in Pennsylvania, which was written into law as I-99 by Pennsylvania Congressman Bud Shuster; I-99 (which is also U.S. Route 220) is west of several Interstates that are numerically less than 99, but 99 was the nearest odd 2-digit number available for the interstate.
A less-notable exception is I-82, which lies fully north of I-84; this is a relic from I-84 previously having the designation of I-80N.
Some Proposed Interstate Highways have been given similarly non-conforming designations by their legislative proponents. For example, backers of the proposed Third Infantry Division Highway, a route in Georgia and Tennessee, have suggested it be named Interstate 3, in honor of the U.S. 3rd Infantry Division.[citation needed]
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