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.
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. 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).

The highways in the system are typically known as Interstate XX or I-XX; sometimes Interstate Highway XX (IH XX) or Interstate Route XX (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.

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. The public controlled-access highways of Puerto Rico are the Autopistas (PR-22, PR-52, and PR-53).



Commemorative sign introduced in 1993.
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 lobbied for by major U.S. automobile manufacturers and championed by President Dwight D. Eisenhower, and was influenced by both his experiences as a young soldier crossing the country in 1919 following the route of the Lincoln Highway, 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.The General location of national system of interstate highways, including all additional routes at urban areas designated in September, 1955 maps what became the interstate system, and is informally known as the Yellow Book.

Although construction on the Interstate Highway system continues, it was officially regarded as complete in 1991 (though 5.6 miles of the original planned route remain either unconstructed, or not yet open [1]). 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

It has been suggested that this article or section be merged into Speed limit. (Discuss)

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)[2]. 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 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.

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

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.

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.

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.

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. This procedure is known as Contra-Flow, and could be seen in the evacuations of New Orleans, La and Houston, Tx prior to hurricanes Katrina and Rita respectively. 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 specifically for contra-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.

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.[3] However, the Germans in World War II used the Autobahns 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 populous island of Oahu. They have the designation of H-X, and connect military bases. 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. As you travel north, the east-west (even) Interstate numbers increase, and as you travel east, the north-south (odd) Interstate numbers increase. 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. However, not all primary routes traverse long distances. I-45 runs from Galveston, Texas north to Dallas, Texas, a distance of only 284 miles. It is the only primary route that does not cross state lines (see List of intrastate Interstate Highways).

It should be noted that I-50 and I-60 do not exist (and there are no even-numbered Interstates between 46 and 62), mainly because they would most likely have passed through the same states that already have US 50 and US 60. AASHTO rules discourage Interstate and US Highways with the same number to exist in the same state, although I-24 and US 24 exist at opposite ends of Illinois. Some planned Interstates do not follow this guideline - I-69 will enter Texas (which has US 69), I-74 will have a multiplex with US 74 in North Carolina, and I-41 will do the same with US 41 in Wisconsin.

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 eliminate 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. I-94 intersects the loop in two spots and runs directly through it separating it into a northern and southern half. The southern half of it is labeled I-494 while the northern half of it is labeled I-694.

Charlotte, North Carolina has a single loop around the city that intersects with both I-77 and I-85, but the entire loop is known as I-485.

The Baltimore-Washington metropolitan area has several spur routes off of I-95. The area has I-195, I-295, I-495, I-795 and I-895. It also has two routes numbered I-395 (in Baltimore and Washington) and two I-695s (one is signed, the other is a secret designation), as well as an unsigned route called I-595. No I-995 exists anywhere.

New York City has numerous spur routes off of I-78 and I-95, but none of I-78's spur routes actually intersect with I-78.

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.


Main article: List of gaps in Interstate Highways

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 (which is also U.S. Highway 220) is west of several Interstates that are numerically less than 99, and was the nearest available unused two-digit number.

Some proposed future Interstate routes have been given similarly non-conforming designations by their legislative proponents. For example, backers of the proposed Third Infantry Division Highway, a route in Georgia and Tennessee, have suggested it be named Interstate 3, in honor of the batallion for which the highway is named [4].

other notable examples

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.


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, and effectively holds funds as ransom in order to coerce state governments into passing laws that would not have otherwised been introduced.

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[5]. 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 Minneapolis, Houston, and Dallas.

Non-Chargeable Interstate routes

In addition to Interstate highways financed with federal funds (Chargeable Interstate routes), federal laws allow other highways to be signed as Interstates, if they meet the Interstate Highway standards and that they are logical additions or connections to the System.

Called Non-Chargeable Interstate routes, these additions fall under two categories:

  1. Routes that already meet Interstate standards. They can immediately be signed as Interstates once their proposed number is approved.
  2. Routes designated as a future part of the system once they are upgraded to Interstate standards. Until then, it cannot be signed as an Interstate yet.


I-95 shield
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. The sign itself measures 36 inches high, and is 36 inches wide for two-digit interstates, or 45 inches for three-digit interstates.

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). Many northeastern states 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 over top 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. It is also purported to be the "world's largest".
  • Kearney, Nebraska — The Great Platte River Road Archway Museum is built over top of Interstate 80. The 1,500 ton structure spans 308 feet across the interstate and houses a museum dedicated to frontier culture.
  • Boston, Massachusetts — The Prudential Tower in downtown Boston is built over top of Interstate 90 as well. A less known fact about Interstate 90 is that it is built over what used to be the largest railroad corridor in New England; photos from the 1950s show huge rail yards where I-90 now comes into the city. Many of the oddities on I-90 are the result of deals struck by the Massachusetts Turnpike Authority, Perini Corporation (which constructed the majority of the highway), and existing buildings at the time of construction. In return for not obstructing the highway project, the MTA went to lengths such as preserving the aforementioned Star Market.
  • Atlanta, Georgia — The Civic Center MARTA station is located over the Downtown Connector (Interstate 75/85) at West Peachtree Street, making it the only subway station built over an Interstate highway.
  • Breezewood, Pennsylvania — There is a sign of a policeman pointing at you saying, "You! Slow Down!" You then have to drive a few blocks on US 30 before returning to I-70. This is a rare instance of a traffic light on an interstate.


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, Charles Kuralt stated, "It is now possible to travel from coast to coast without seeing anything!"


^  Field, David. "On 40th birthday, interstates face expensive midlife crisis." Insight on the News, 29 July 1996, 40-42.

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 (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

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