Everything about Interstate Highway System totally explained
The
Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the
Interstate Highway System (or simply, the
Interstate System), is a network of
highways (also called freeways or expressways) in the
United States that's named for
the president who was in office and championed its creation. 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), making it both the largest highway system in the world and the largest
public works project in history.
While Interstate highways usually receive substantial federal funding and comply with federal standards, they're owned, built, and operated by the states or toll authorities. The original
Woodrow Wilson Bridge, for example, 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.
The system serves nearly all major U.S. cities, with many Interstates passing through
downtown areas. The distribution of virtually all goods and services involves Interstate highways at some point. 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; of these trips, about one-third (by the total number of miles driven in the country in 2003) utilize the Interstate system.
History
The Interstate Highway 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 his experiences in 1919 as a young soldier crossing the country (following the route of the
Lincoln Highway) and his appreciation of the
German autobahn network as a necessary component of a national defense system. In addition to facilitating private and commercial transportation, it would provide key ground transport routes for military supplies and troop deployments in an emergency.
Initial federal planning for a nationwide highway system began in 1921 when the
Bureau of Public Roads asked the
Army to provide a list of roads it considered necessary for national defense, resulting in the
Pershing Map. Later that decade, highways such as the
New York parkway system had been built as part of local or state highway systems. As automobile 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.
Although construction on the Interstate Highway System continues,
I-70 through
Glenwood Canyon, completed in 1992, is often cited as the completion of the system. The initial cost estimate for the system was $25 billion over 12 years; it ended up costing $114 billion (adjusted for inflation, $425 billion in 2006 dollars) and taking 35 years to complete.
Due to the cancellation of the
Somerset Freeway,
Interstate 95 is discontinuous in
New Jersey. When the
Pennsylvania Turnpike/Interstate 95 Interchange Project concludes in or around 2009, the last section of the original plan will be completed.
First Interstate highway
Three different states have laid claim to the title of first Interstate highway. Missouri claims that the first three contracts under the new program were signed in Missouri on
August 2,
1956. The first contract signed was for U.S. 66 (now
I-44). On
August 13,
1956, Missouri awarded the first contract based on new Interstate highway funding.
Kansas claims that it was the first to start paving after the act was signed. Preliminary construction had taken place before the act was signed, and paving started
September 26,
1956. The state marked its portion of I-70 as the "first project in the United States completed under the provisions of the new Federal-Aid Highway Act of 1956."
Other uses
As one of the components of the National Highway System, Interstate highways improve the mobility of military troops to and from airports, seaports, rail terminals and other military bases. Interstate highways also connect to other roads that are a part of the
Strategic Highway Network (abbreviated STRAHNET), a system of roads identified as critical to the
U.S. Department of Defense.
The system has also been used to facilitate evacuations in the face of hurricanes 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, has been employed several times for hurricane evacuations. After public outcry regarding the inefficiency of evacuating from southern Louisiana prior to
Hurricane Georges' landfall in September 1998, government officials looked towards contraflow to improve evacuation times. In
Savannah, Georgia and
Charleston, South Carolina in 1999, lanes of Interstates
16 and
26 were used in a contraflow configuration in anticipation of
Hurricane Floyd, with mixed results. 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; however, evacuation times there were no better than previous evacuation operations. Engineers began to apply lessons learned from the analysis of prior contraflow operations, including limiting exits, removing troopers (so as to keep traffic flowing, instead of having drivers stop for directions), and improving the dissemination of public information. As a result, the 2005 evacuations of
New Orleans, Louisiana, and
Houston, Texas, prior to hurricanes
Katrina and
Rita ran far more smoothly.
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.
Terminology
While the name implies that Interstate highways cross state lines, many don't (for details see
List of intrastate Interstates). Rather, they're
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-
and connect military bases, though they're open to public use. Both
Alaska and
Puerto Rico have public roads that receive funding from the Interstate program, although these routes are not signed as Interstate highways (except on paper). These roads are neither planned for, nor built to, official Interstate highway standards.
Primary routes
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 Western New York, Southern California, Southern Nevada, and Honolulu,
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). The association's latest numbering policy is unchanged from policy dated to
August 10,
1973. Within the continental United States, primary Interstates (also called main line Interstates or two-digit Interstates) are assigned numbers less than 100. AASHTO policy discourages Interstate and U.S. highways having the same number within the same state, although there are a few exceptions. Major north–south arterial Interstates increase in number from
I-5 between
Canada and
Mexico along the west coast to
I-95 between
Miami and
Canada along the east coast. Major east–west arterial Interstates increase in number from
I-10 between
Santa Monica, California and
Jacksonville, Florida to
I-90 between
Seattle and
Boston. Two-digit Interstates in
Hawaii, as well as the "paper" Interstates of
Alaska and
Puerto Rico, are numbered sequentially in order of funding, without regard to the rules on odd and even numbers.
Several two-digit numbers are shared between two roads at opposite ends of the country. Some of these were the result of a change in the numbering system as a result of the new policy adopted in 1973. 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. The new policy stated that "No new divided numbers (such as
I-35W and
I-35E, etc.) shall be adopted." The new policy also stated recommended that existing divided numbers be eliminated as quickly as possible; however, I-35W and I-35E still exist in
Minnesota and
Texas. Some auxiliary highways don't follow these guidelines, however. See
List of auxiliary Interstate Highways for these examples.
In the example to the right, City A has an even-numbered circumferential highway. City B has an even-numbered circumferential beltway and an odd-numbered spur. City C has an even-numbered circumferential highway and an odd numbered spur. Because cities A, B and C are in the same state, each auxiliary route carries a distinct three-digit route number.
Business Routes, also known as
Business Loops and
Business Spurs are routes that principally travel through the corporate limits of a city, passing through the
central business district of the city. Business routes are used when the regular route is directed around the city. This has led to the proliferation of toll roads (turnpikes) as the new method of building limited-access highways in suburban areas. Also, some Interstates are being privately maintained (for example, VMS maintains I-35 in Texas) to meet rising costs of maintenance and allow state departments of transportation to focus on serving the fastest growing regions in their respective states.
It is possible that parts of the system will have to be tolled in the future to meet maintenance and expansion demands, as has been done with adding toll HOV/
HOT lanes in certain cities such as
San Diego,
Salt Lake City,
Minneapolis,
Houston,
Dallas,
Atlanta and
Washington, D.C.
Local maintenance
A few Interstates are maintained by local authorities:
Chargeable and non-chargeable Interstate routes
Interstate highways financed with federal funds are known as "chargeable" Interstate routes, and are considered part of the network of highways. Federal laws allow highways funded similarly to state and US highways to be signed as Interstates, if they meet the Interstate highway standards and are logical additions or connections to the System.
Called "non-chargeable" Interstate routes, these additions fall under two categories: routes that already meet Interstate standards, and routes not yet upgraded to Interstate standards. Only routes that meet Interstate standards may be signed as Interstates once their proposed number is approved.
Signage
The majority of Interstates have
exit numbers. All
traffic signs and
lane markings on the Interstates are supposed to be designed in compliance with the
Manual on Uniform Traffic Control Devices (MUTCD). However, there are many local and regional variations in signage.
For many years,
California was the only state that didn't use an exit numbering system. It was granted an exemption in the 1950s due to having an already largely completed and signed highway system; at the time, placing exit number signage across the state was deemed too expensive. Since 2002, however, California has begun to incorporate exit numbers on all its freeways - Interstate, U.S., and state routes alike. To mitigate costs, a common occurrence is for
Caltrans to install exit number signage only when a freeway or interchange is built, reconstructed, retrofitted, or repaired. The majority of the exits along California's Interstates now have exit number signage, particularly in rural areas.
In most states, the exit numbers correspond to the mileage markers on the Interstates. However, on
I-19 in
Arizona, length is measured in kilometers instead of miles, in part because it runs south to the Mexican border. On most even-numbered Interstates, mileage count increases from west to east; on odd-numbered Interstates, mileage count increases from south to north. Some tollways, including the
New York State Thruway and
Jane Addams Memorial Tollway, use radial exit numbering schemes. Exits on the New York State Thruway count up from
Yonkers traveling west. On the Jane Addams Memorial Tollway, mileage markers count up from
Chicago-O'Hare International Airport traveling west.
Many northeastern states label exit numbers sequentially, regardless of how many miles have passed between exits. States in which Interstate exits are still numbered sequentially are Connecticut, Delaware, Massachusetts, New Hampshire, New York, Rhode Island and Vermont. Maine, Virginia, Georgia, and Florida followed this system for a number of years, but recently converted to having the exit numbers correspond to mileage markers. The
Pennsylvania Turnpike uses both the mile marker number and the sequential number. The mile marker number is used for signage, while the sequential number is used for numbering interchanges internally. The New Jersey Turnpike also has sequential numbering, but other Interstates within New Jersey generally use mile markers.
Interstate shield
Interstate Highways are signed by a number placed on a trademarked
Interstate
business loops and
spurs use a special shield where the red and blue are replaced with green, the word BUSINESS appears instead of INTERSTATE, and the word SPUR or LOOP usually appears above the number.
Over time the design of the Interstate shield has changed. In 1958, when the Interstate shield was introduced, the shield color was a dark navy blue and only 17-in (41 cm) wide. The least traveled section is
Interstate 95 just north of
Houlton, Maine (near the Canadian border), with 1,880 vehicles a day (2001 estimate).
The most extreme directional points of the Interstate Highway system are:
Northernmost: The northern termini of Interstates 5, 15, and 29, crossing the Canadian border at the 49th parallel near, respectively, Blaine, Washington, Sweetgrass, Montana, and Pembina, North Dakota.
Southernmost: A bend on Interstate H-1 in the Kaimuki section of Honolulu, Hawaii, less than 1/2 mile (0.8 km) before its eastern terminus (lat. 21.3 deg. N.). The southernmost point in the 48 contiguous states is the southern terminus of Interstate 95 in Miami, Florida (lat. 25.8 deg. N.).
Easternmost: The northern terminus of Interstate 95 near Houlton, Maine, at the border of New Brunswick in Canada (long. 67.8 deg. W.).
Westernmost: The western terminus of Interstate H-1 in Kapolei, Hawaii (long. 158.06 deg. W.). The westernmost point in the contiguous states is a curve on Interstate 5 near Wolf Creek, Oregon (long. 123.23 deg. W.).
The highest point on the Interstate Highway System is at the Eisenhower Tunnel on Interstate 70 in the Rocky Mountains of Colorado, at the Continental Divide (elev. ). The lowest point on land is on Interstate 8 at the New River near Seeley, California (elev. ). The lowest point under water is on Interstate 95 in the Fort McHenry Tunnel under Baltimore Harbor in Baltimore, Maryland (elev. ).
The longest Interstate highway is Interstate 90, which runs between Boston, Massachusetts and Seattle, Washington. The longest north-south Interstate highway is generally cited as Interstate 95; when completed, it'll run between Miami, Florida and the Canadian border (there is a gap in New Jersey). The shortest, albeit unsigned, Interstate is Interstate 878, a portion of New York State Route 878 adjacent to John F. Kennedy International Airport in Queens, New York. Another short unsigned Interstate is Interstate 110 near downtown El Paso, with a length of . The shortest signed Interstate is Interstate 375 in downtown Detroit, Michigan, at .
The shortest Interstate route segment within a state (or federal district) is I-95 in the District of Columbia which is long.
Further Information
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