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Interstate highway

The Dwight D. Eisenhower System of Interstate and National 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 . The system was modeled after the German autobahn system and started under President Eisenhower in 1956.

Commemorative sign introduced in 1993

While the name implies highways that cross state lines, many Interstates don't. 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 Hawaii. (Although there are no controlled-access Interstate highways in Alaska, there are other public roads in Alaska that receive funding from the Interstate program.)

Table of contents
1 Financing
2 Physical design
3 Signage
4 Naming of the highways
5 See also
6 External links


Often, depending on the part of the country, these roads are called freeways. The term is also used to describe toll-free superhighways, but can sometimes also be used to describe such roads whether there is a toll or not, because they are free-flowing. 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.

Physical design

With few exceptions (which may be discussed as part of information on the individual highways when known, but will not be further acknowledged in this article), Interstate highways meet controlled-access specifications. Thus they do not have at-grade intersections, but instead all interchanges and other intersections use overpasses and underpasses. All access to the highway is via exit and entrance ramps that reduce the interferance with the flow of through traffic. Traffic lights are limited to toll booths (and toll booths are limited to grandfathereded roads and bridges), draw bridges, and ramp meters (metered flow control) for lane merging during rush hours. 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 except in limited stretches (such as steep mountain passes) where many vehicles cannot maintain such speeds.


Marker for Interstate Highway 95
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 the business district without meeting the specifications for Interstate highways.

Naming of the highways

The numbering scheme for the Interstate highway system is administered by the American Association of State Highway and Transportation Officials(AASHTO). Major Interstate highways are given one- or two-digit 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 number and "north" for the other.) Odd numbers increase from west to east; and even numbers increase from south to north. For example, I-80 connects San Francisco in the west and Fort Lee, New Jersey in the east. I-5 goes from Canada at its north end to Mexico at its south one, along the west coast.

Three digit numbers, consisting of a single digit prefixed to the number of a major Interstate highway, are used to designate highway extensions, spurs (odd prefixes), and bypasses (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). These three-digit numbers may be repeated in different states for different roads. Interstate 238 near Oakland, California is the lone exception to the numbering scheme, as no Interstate 38 exists (this number exists because Interstate 238 replaced a segment of California Highway 238 and changing the number would have split the California Highway in two segments. The original number was retained to keep the California Highway contiguously numbered).

Some old local highways may be renamed when included in the federal system. For example, part of California highway 17 connecting Oakland and San Jose was renamed as I-880 in the mid 1980s. Part of the original California Highway 17 still connects San Jose and Santa Cruz.

See also

External links