Interstate Highway

The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the Interstate Highway System, is a network of highways in the United States. The Interstate Highway System is a separate system within the larger National Highway System. With very few exceptions, Interstate highways are controlled-access freeways, allowing for safe high-speed driving when traffic permits. They are assigned a special level of funding at the federal level. Despite this federal funding, these highways are owned, designed, built and maintained by the state in which they are located, with the only exception being the federally-owned Woodrow Wilson Bridge on the Capital Beltway (I-95/I-495).

Standards

:Main article: Interstate Highway standards

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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 stricter over the years. One almost absolute standard is the controlled access nature of the roads. Except for a 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 hours).

Related Topics:
American Association of State Highway and Transportation Officials - Federal Highway Administration - Controlled access - Exceptions - Traffic light - Toll booth - Ramp meter - Rush hour

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Speed limits

Speed limits vary according to location. By initial planning, the Interstate system was designed to provide reasonable road safety at speeds of 75 to 80 miles per hour (120 to 130 km/h) except in limited stretches (such as steep mountain passes or urban cores) where many vehicles cannot maintain such speeds. Many western states had high speed limits. Kansas, for example, had a posted limit of 80 mph (130 km/h)http://people.smu.edu/acambre/blog/PermaLink,guid,5575625c-bbd4-47a3-be9f-9d5517d3a06f.aspx. Some states, such as Oregon, defined the limit as whatever was "reasonable and proper", which would not be allowed today (see Montana reference below).

Related Topics:
Speed limit - Road safety - Mountain pass - Kansas - Oregon - Montana

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In 1974, the federal government enacted 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 as a safety measure. It was very unpopular, especially in western states. The 55 mph cap was relaxed in 1987 to allow 65 mph (105 km/h) speeds on rural interstates areas if the states so chose. Shortly thereafter, 65 mph limits were allowed on roads not numbered as interstates but which were built to interstate standards.

Related Topics:
1974 - 55 mph - Gasoline - Conservation - 1973 energy crisis - Western states - 1987

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The 55/65 mph caps were eliminated in late 1995, fully returning speed limit control to the states.

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Many states maintain several different limits. 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. In some states, commercial trucks have a lower speed limit than passenger automobiles. In some mountainous regions, the condition of the roadway mandates a lower speed limit than would otherwise have applied.

Related Topics:
California - Suburban - Rural

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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). Generally, the highest speed limits are found in the South and Southwest, while the lowest are found in the Northeast. Soon after the end of the National Maximum Speed Limit, the state of Montana ended daytime speed limits for automobile traffic on Interstate Highways in the state, instead instructing motorists to maintain a "reasonable and prudent" speed. A few years later, the "reasonable and prudent" law was declared unconstitutional for being too vague and a limit of 75 mph (120 km/h) was enacted in its place.

Related Topics:
South - Southwest - Northeast - National Maximum Speed Limit - Montana - Unconstitutional

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Texas recently enacted a law allowing 80 MPH speed limits on certain portions of Interstates 10 and 20 in far west Texas. However, these limits are on hold pending further study by the Texas Department of Transportation.

Related Topics:
80 MPH speed limits - Texas Department of Transportation

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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.

Related Topics:
Military - Civil defense

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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. 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. This is however not limited to Interstates; State Road 528 has the same setup in central Florida.

Related Topics:
Civil defense - Evacuation - Nuclear war - Hurricanes - South U.S. - I-16 - Georgia - I-40 - North Carolina - I-65 - Alabama - I-10 - Louisiana - State Road 528 - Central Florida

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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.http://www.tfhrc.gov/pubrds/mayjun00/onemileinfive.htm However, the Germans in World War II used the Autobahns for just such a purpose.

Related Topics:
Urban legend - World War II - Autobahn

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