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Encyclopedia > Interstates
A typical rural stretch of Interstate highway, with two lanes in each direction separated by a large grassy median, and with cross-traffic limited to overpasses and underpasses.
Commemorative sign introduced in 1993

The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the interstate highway system, is a network in the United States of interstate highways or simply interstates. Nearly every interstate highway is a controlled-access superhighway or freeway allowing for safe high-speed driving when traffic permits. There are a few exceptions however. The unsigned Interstate highways of Puerto Rico (PR-1, PR-2, and PR-3) are not entirely controlled-access. The superhighways of Puerto Rico are the "Autopistas" (PR-22, PR-52, and PR-53), which are not funded by the Eisenhower Interstate System.

There are a few remaining non-freeway segments which cannot be upgraded for political or financial reasons; see List of gaps in Interstate Highways.

The system serves practically all major U.S. cities, and unlike its counterparts in most industrialized countries, often goes right into downtown areas rather than bypassing them.

It is prominent in the daily lives of most Americans. Virtually all goods and services are delivered via the interstate highways at some point. Most residents of American cities use the urban segments of the system to go to and from their jobs. Most long-distance journeys (for vacation or business) of less than 300 miles (483 km) use the interstate highway system at some point. For longer journeys, travel is more often by airplane.



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 was championed by President Dwight D. Eisenhower, and was influenced by both his experiences as a young soldier crossing the country in 1919 and by his appreciation of the German autobahn network.

Planning for a system of new "superhighways" began in the late 1930s, even before federal commitment to build the Interstate highway system came in the 1950s. Construction on the world's first limited-access highway, the Bronx River Parkway, had begun in New York as early as 1907. By the 1920s, longer highways such as the New York City 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 highway system.

Although construction on the Interstate Highway system continues, it was regarded as complete in 1991. The initial cost estimate for the system was $25 billion over twelve years; it ended up costing $114 billion, taking 35 years to complete. As of 2004, the system contains over 42,700 miles (68,500 km) of roads.


While the name implies highways that cross U.S. state lines, many interstates do not. Rather, it is the system of interstates that connects states. There are interstate highways in Hawaii, funded in the same way as in the other states, but entirely within the islands of Hawaii. 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.

See also List of intrastate Interstate Highways.


Often, depending on the part of the country, these roads are called freeways. The term is taken by many to describe toll-free superhighways, but originally meant that abutting property owners did not have access rights to the road; access is only at on/off-ramps. Almost all of the construction and maintenance cost is funded through user fees, primarily gasoline taxes, collected by states and the federal government, and tolls collected on toll roads and bridges. In the eastern United States, sections of some Interstate highways planned or built prior to 1956 are operated as toll roads, and are often called turnpikes.

The dominant role of the federal government in road finance has enabled it to pass laws in areas outside of the powers enumerated in the federal Constitution. By threatening to withhold highway funds, the federal government has been able to force state legislatures to pass a variety of laws. Examples include increasing the legal drinking age to 21, for a number of years reducing the maximum speed limit to 55 miles per hour, passing Megan's Law legislation, lowering the legal intoxication level to 0.08/1000, and other laws. This has proved to be controversial. Those who support this feel that it is a way to provide an impetus to states to pass uniform legislation. Others feel that using highway dollars in this fashion upsets the balance between federal and states' rights in favor of the federal government.

Speed limits

Speed limits vary according to location. By initial planning, the Interstate system was designed to be able to move traffic at speeds of 75 to 80 miles per hour (120 to 130 km/h) except in limited stretches (such as steep mountain passes) where many vehicles cannot maintain such speeds. In 1974, the maximum speed limit allowed on interstate highways (along with all others in the country) was reduced to 55 mph (90 km/h) as a gasoline conservation measure in response to the 1973 energy crisis. After the end of the embargo this restriction was continued, being justified as a safety measure, but it was very unpopular, especially in western states; it was relaxed in 1987 to allow 65 mph (100 km/h) speeds in most areas if the states so chose, and eliminated in 1995, fully returning speed limits to state control. While some states have maintained the 65 mph limit, other states have increased the limits to 70 or 75 mph (110 or 120 km/h). Soon after the end of Federal speed limits, the state of Montana ended speed limits for automobile traffic on interstate highways in the state, instead instructing motorists to maintain a "reasonable and prudent" speed on interstates during the daytime hours. A few years later, the "reasonable and prudent" law was declared unconstitutional for being too vague. A limit of 75 mph was enacted in its place.

Traffic lights are limited to toll booths (and toll booths are limited to grandfathered roads and bridges), draw bridges, and ramp meters (metered flow control) for lane merging during rush hours.

Dual purpose design

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.

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. Although this use has never happened, the Interstate highway system has been used to facilitate evacuations in the face of hurricanes and other natural disasters. An option for maximizing throughput is to reverse the flow of traffic on one side so that all lanes become outbound lanes. Interstate 16 west from Savannah, Georgia is equipped and signed for reverse flow. A number of miles inland is a crossover to route traffic back to the correct side.

A widespread but false 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.[1] (http://www.tfhrc.gov/pubrds/mayjun00/onemileinfive.htm)


On maps and the road, the highway is indicated by a number on a red, white and blue sign in a shape of a shield. On signs on the side of the road, the current state was formerly listed above the highway number, but in some states this area is now left blank. The Interstate shield is not to be confused with the similar-shaped green shield for a business route or loop, which gives access to and from a business district without meeting the specifications for Interstate highways.

Naming of the highways

Primary routes

The numbering scheme for the Interstate highway system is administered by the American Association of State Highway and Transportation Officials (AASHTO). Within the continental United States, Primary Interstate highways (or main line interstates) are given one- or two-digit route numbers. Within this category, even-numbered highways go generally east-west, and odd-numbered highways go generally north-south. (However, in some places two or more Interstate highways run along the same physical road, and such a road may be "east" for one route and "north" for the other.) Odd numbered routes increase from west to east; and even numbered routes increase from south to north. Numbers divisible by 5 are roads which go all the way (or nearly all the way) from one border of the U.S. to another. For example, I-5 runs from Canada to Mexico along the west coast while I-95 runs from Miami north to Canada. In addition, I-10 runs from Los Angeles, California to Jacksonville, Florida while I-80 runs from San Francisco to Fort Lee, New Jersey. There is also an I-90 that runs from Seattle to Boston, and shares a portion of its routing with I-80.


Three-digit route numbers, consisting of a single digit prefixed to the number of a primary Interstate highway, are used to designate highway extensions, spurs (odd prefixes), and bypasses and loops (even prefixes) that connect to the main highway within an urban area. For example, there are many extensions to I-80 in the San Francisco Bay Area: I-280 connects San Francisco and San Jose; I-380, I-580, I-680, I-780, I-880, I-980 are also major highways. (I-480 was also an extension before it was demolished following local popular opposition). Three-digit route numbers may be repeated in different states for different roads.


Interstate 238 near Oakland, California is one of two exceptions to the numbering scheme, as no Interstate 38 exists. (This number exists because Interstate 238 replaced a segment of California Highway 238, and no appropriate number was available.) The other exception is I-99 in Pennsylvania which was written into law as I-99 by Pennsylvania Congressman Bud Shuster, as it is west of several Interstates that are numerically less than 99, and was the nearest unused two-digit number.

Main line 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 above.

See also List of gaps in Interstate Highways.

See also

Primary Interstate Highways Interstate Highway marker
4 5 8 10 12 15 16 17
19 20 22 24 25 26 27 29
30 35 37 39 40 43 44 45
49 55 57 59 64 65 66 68
69 70 71 72 73 74 75 76
77 78 79 80 81 82 83 84
85 86 87 88 89 90 91 93
94 95 96 97 99 238
H-1 H-2 H-3
Unsigned Interstate Highways
A-1 A-2 A-3 A-4 PRI-1 PRI-2 PRI-3
All Interstates - Gaps in Interstates - Intrastate Interstates

External links

  Results from FactBites:
Interstate Highway - Wikipedia, the free encyclopedia (3844 words)
For example, in California, most interstates are limited to 55 mph within a major city, 65 mph (105 km/h) for most of the suburban highway stretches, and up to 70 mph (115 km/h) throughout the desert and rural stretches of the state.
Several Interstates in the South U.S., including I-16 in Georgia, I-40 in North Carolina, I-65 in Alabama, and I-10 in Louisiana, are equipped and signed for reverse flow, with crossovers inland after major interchanges to distribute much of the traffic.
On even-numbered Interstates, mileage increases to the east and decreases to the west; and on odd-numbered Interstates, mileage increases to the north and decreases to the south.
  More results at FactBites »



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