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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 of controlled access freeways in the United States. The system was modelled after the German autobahn system and started under President Dwight Eisenhower in 1956.

Although Federal Government commitment to build the Interstate Highway system did not come until the 1950s, planning for a new system of "superhighways" began in the late 1930s. The United States already had a number of limited access highways, such as the Pennsylvania Turnpike and the New York Parkway system dating back to the 1920s (construction on the world's first limited access highway, the Bronx River Parkway, was begun in New York State in 1907), but these were either local or state highways, and there was no interconnected national system. A need was seen for such a system to to supplement the existing largely at-grade United States highway system.

Although construction on the Interstate Highway system is ongoing, it was regarded as complete in 1990.

While the name implies highways that cross state lines, many highways don't. Rather, it is the system of highways that connects states. There are interstate highways in Hawaii, funded in the same way as in the other states, but entirely within Hawaii. (Alhough there are no freeways in the system in Alaska, there are other public roads in Alaska that receive funding from the Interstate program).

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The roads are called freeways. The term is usually 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 the gas tax[?], collected by states and the federal government, and tolls collected on toll roads and bridges. In the eastern US, sections of some interstate highways planned or built prior to 1956 are operated as toll roads, and are often called turnpikes.


Interstate highways normally do not have at-grade intersections (there a few exceptions, listed with the individual highways). All intersections use overpasses, underpasses, and interchanges. All access to the highway is via access ramps that do not interfere with the flow of traffic. Traffic lights are limited to toll booths (and toll booths are limited to grandfathered roads and bridges), draw bridges, and ramp meters or metered flow control for lane merging during rush hours. Speed limits vary according to location. By initial planning, the Interstate system was desinged 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 this would not be practical.


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.

Naming of Interstate 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 Teaneck, New Jersey in the east. I-5 goes from Washington state in the north to Mexico in the south 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, and bypasses 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, so the number was retained to keep the California Highway contigiously 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.

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