Interstate Highway

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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.

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. With very few exceptions, these 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).

The highways in the system are typically known as Interstate XX or I-XX; sometimes Interstate Highway XX (IH XX) or Interstate Route (IR XX) is used. In some areas the more generic Route XX or Highway XX is used. The system serves practically all major U.S. cities, and unlike its counterparts in most industrialized countries, often goes right through downtown areas rather than bypassing them. This facilitated the emergence of automobile-oriented postwar suburban development patterns, often pejoratively referred to as "urban sprawl".

The system is prominent in the daily lives of most Americans. Virtually all goods and services are delivered via the Interstate Highways at some point. Many 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 (500 km) use the interstate highway system at some point. For longer journeys, travel is done more often by airplane.

Hawaii has several signed Interstates, but Alaska and Puerto Rico do not. The latter two do have roads designated as Interstates for funding purposes, but they are not currently or planned to be built to Interstate standards. To the public, the controlled-access highways of Puerto Rico are the Autopistas (PR-22, PR-52, and PR-53).



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Commemorative sign introduced in 1993.

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 public 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, U.S. Highway system.

Although construction on the Interstate Highway system continues, it was officially 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, all at least four lanes wide.


Main article: Interstate Highway standards

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).

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. Many western states had high speed limits. Kansas, for example, had a posted limit of 80 mph (130 km/h). Some states, such as Oregon, defined the limit as whatever was "reasonable and proper", which would not be allowed today (see Montana reference below).

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 (105 km/h) speeds in most areas if the states so chose, and eliminated in 1995, fully returning speed limits to state control. During this interim period, some roads (such as I-88 in Illinois) were specifically designated as Interstates to take advantage of this higher speed limit.

With the elimination of the 55 mph (90 km/h) limit, power reverted to the individual states to set their speed limits. Many states, such as California and Virginia, maintain several different limits - within Los Angeles, most interstates are limited to 55 mph within the city, and 65 mph for most of the suburban highway stretches, and up to 70 mph throughout the desert and farmland rural stretches of the state. In some states, such as California and Indiana, commercial trucks have a separate, lower speed limit than the limit that applies to normal automobiles. California sets a state-wide limit on truck speeds at 55 mph, irrespective of the automobile speed limit. In other places, such as Wheeling, West Virginia, on Interstate 70, the condition of the roadway mandates a lower speed limit than would otherwise have applied.

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 Minimum 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. A limit of 75 mph (120 km/h) was enacted in its place.

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

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] ( The Germans in World War II did use the Autobahnen for just such a purpose.


While the name implies that these highways cross 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.

Primary routes

The numbering scheme for the Interstate Highway System (as well as the U.S. Highway System) is coordinated by the American Association of State Highway and Transportation Officials (AASHTO), though their authority is occasionally trumped by a number written into Federal law. Within the continental United States, primary Interstates (also called main line Interstates or two-digit Interstates) are given one- or two-digit route numbers. Within this category, even-numbered highways are signed east-west, and odd-numbered highways are signed north-south. Odd numbered route numbers increase from west to east, and even numbered routes increase from south to north. Numbers divisible by 5 are intended to be primary routes, carrying traffic long distances. 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-90 runs from Seattle to Boston.

Several two-digit numbers are shared between two roads at opposite ends of the country, namely I-76, I-84, I-86 and I-88. Some of these were the result of a change in the numbering system in the 1970s; 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. In the 1970s, AASHTO decided to get rid of these; some became additional two-digit routes, while others became three-digit routes (see below). Only two pairs of these exist; I-35 splits into I-35W and I-35E through both the Dallas-Fort Worth and the Minneapolis-St. Paul areas.

Strict adherence to the directional nature of the system results in some amusing oddities. For a ten-mile stretch east of Wytheville, Virginia, the driver can be traveling on both North I-81 and South I-77 at the same time (and vice versa) (see also Wrong-way multiplex).

For the sake of efficiency, some Interstates double up for short or sometimes long distances. The above is one example. Another notable example is I-90 and I-94, which double and then separate several times as they criss-cross the upper Midwest and Great Plains.

Three-digit Interstates

Three-digit route numbers, consisting of a single digit prefixed to the number of a primary Interstate highway, are used to designate usually short spur or loop routes from their "parent" route, either directly or via another three-digit Interstate. A route that spurs from its parent and ends at an intersection with no other Interstates is given an odd first digit; a route that returns to its parent is given an even first digit. The number given to the first digit of a route that spurs from the parent and ends at another Interstate depends on the state; some consider these routes spurs and give them odd numbers, while others consider them loop-style connectors and give them even numbers.

For instance, I-90 in New York has a full set of three-digit Interstates - I-190, I-290, I-390, I-490, I-590, I-690, I-790, I-890 and I-990. Due to the large number of these routes, they can be repeated in different places along the mainline; no two three-digit Interstates in the same state can share a number.

The Minneapolis-St. Paul area has a single loop around the entire Metro area, but the southern half of it is labeled I-494 while the northern half of it is labeled I-694.

A three-digit spur off a letter-suffixed two-digit Interstate (see above) was given a number without a letter suffix, except for one case - I-184 in Idaho was I-180N.


Interstate 238 near Oakland, California is one of two major 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; I-99 is west of several Interstates that are numerically less than 99, and was the nearest available unused two-digit number.

I-82 lies fully north of I-84, but I-84 was I-80N when I-82 got its number.

The following two-digit Interstates change direction from their normal (even=east-west, odd=north-south) direction:

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.

Business Loop and Business Spur Interstates are not subject to any of the Interstate standards. Their designation is simple - a Business Loop heads into a downtown area from its parent and returns to its parent; a Business Spur ends downtown, occasionally continuing from the end of the main Interstate. Business routes can split from either two- or three-digit Interstates, and can be repeated within a state. In a few cases, where an Interstate has been realigned, the old road has been designated a Business Loop because it is not up to standards.

See also list of gaps in Interstate Highways.


About 72% (2003 FHWA summary ( of the construction and maintenance costs are funded through user fees, primarily gasoline taxes, collected by states and the federal government, and tolls collected on toll roads and bridges. The rest of the costs come out of the federal budget. In the eastern United States, large sections of some Interstate Highways planned or built prior to 1956 are operated as toll roads. The taxes dedicated to the construction and maintenance of highways are often criticized as a direct subsidy from the government to promote and maintain auto-oriented development as we know it today.

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.

As American suburbs push ever outward, the costs incurred of maintaining freeway infrastructure has started to catch up with the economy, leaving little in the way of funds for new interstate construction. This has led to the proliferation of the toll road (turnpike) as the new method of building limited-access highways in suburban areas. Also, some interstates are being privately maintained now (VMS in Texas, I-35) in order to cut rising costs of maintenance and allow state departments of transportation to focus on serving the fastest growing regions in their respective states. The future of the interstate system as we know it is in question. It is entirely possible that parts of the system will have to be tolled in the future to meet maintenance and expansion demands, as is done with adding toll HOV/HOT lanes in certain cities like Houston and Dallas.


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I-95 shield

Interstate Highways are signed by a number on a red, white and blue sign as shown to the right. In the original design, the state was formerly listed above the highway number, but in many states, this area is now left blank.

Business Loop and Business Spur Interstates 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.

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. The state of California is now adapting to an exit numbering system after many years as being the only state in the country that did not use such system.

In most states, the exit numbers correspond to the mileage markers on the Interstates (with an exception being I-19 in Arizona, whose length is measured in kilometers instead of miles). Some states such as Connecticut and Florida label exit numbers sequentially, regardless of how many miles have passed between exits. 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. In both cases, the exit numbers increase and decrease accordingly.

Interstate Oddities

  • Vinita, Oklahoma a McDonalds is built overtop of Interstate 44. It goes from one side of the interstate to the other, passing over the interstate. Customers can sit inside and eat while traffic drives beneath them.
  • Kearney, Nebraska The Great Platte River Road, Archway Musuem is built over top of Interstate 80 The 1,500 ton structure spans 308 feet across the interstate and houses a musuem dedicated to frontier culture.


In addition to the various economic issues, the system has roused criticism on aesthetic grounds. The efficiency and faster speeds of the system, made possible in part by engineering techniques that often tend to cut through the land rather than merely following it as with the older U.S. Highway, have inevitably resulted in a safer but less-scenic drive. When the cross-country I-40 was finally finished in the late 1980s, by completing the segment between Raleigh, North Carolina and Wilmington, North Carolina, one newspaper writer stated, "It is now possible to travel from coast to coast without seeing anything!"

See also

Template:Ed divPrimary Interstate Highways Missing image
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 (W)
76 (E) 77 78 79 80 81 82 83
84 (W) 84 (E) 85 86 (W) 86 (E) 87 88 (W) 88 (E)
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
Two-digit Interstates - Three-digit Interstates
Gaps in Interstates - Intrastate Interstates
Interstate standards - Proposed Interstates

External links


eo:Intersxtata Sxosea Sistemo ja:ドワイト・デーヴィッド・アイゼンハワー全米州間国防高速道路網


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