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Design

 
 

Interstate System

Dwight D. Eisenhower National System of Interstate and Defense Highways

Note: This document contains dated information. It is in the process of being updated.

Origins

Planning for what is now known as the Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called "The Interstate System," began in the late 1930's. The Federal-Aid Highway Act of 1938 called on the Bureau of Public Roads (BPR), the predecessor of the Federal Highway Administration (FHWA), to study the feasibility of a toll-financed system of three east-west and three north-south superhighways. The BPR's report, Toll Roads and Free Roads, demonstrated that a toll network would not be self-supporting. Instead, the BPR's report advocated a 26,700-mile interregional highway network.

In 1941, President Franklin D. Roosevelt appointed a National Interregional Highway Committee, headed by Commissioner of Public Roads Thomas H. MacDonald, to evaluate the need for a national expressway system. The committee's January 1944 report, Interregional Highways, supported a system of 33,900 miles, plus an additional 5,000 miles of auxiliary urban routes.

Designation of the Interstate System

In the Federal-Aid Highway Act of 1944, the Congress acted on these recommendations. The act called for designation of a National System of Interstate Highways, to include up to 40,000 miles "... so located, as to connect by routes, direct as practical, the principal metropolitan areas, cities, and industrial centers, to serve the National Defense, and to connect at suitable points, routes of continental importance in the Dominion of Canada and the Republic of Mexico."

On August 2, 1947, Commissioner MacDonald and Federal Works Administrator Philip B. Fleming announced selection of the first 37,700 miles. The routes had been proposed by the State highway agencies and reviewed by the Department of Defense. However, neither the 1944 act nor later legislation in the 1940's authorized funds specifically for the Interstate System. As a result, progress on construction was slow.

Interstate Funding

The Federal-Aid Highway Act of 1952 authorized the first funding specifically for System construction, but it was only a token amount of $25 million a year for fiscal years (FY) 1954 and 1955. Legislation in 1954 authorized an additional $175 million annually for FY 1956 and 1957.

Under the leadership of President Eisenhower, the question of how to fund the Interstate System was resolved with enactment of the Federal-Aid Highway Act of 1956. It served as a catalyst for the System's development and, ultimately, its completion. Title I of the 1956 Act increased the System's proposed length to 41,000 miles. It also called for nationwide standards for design of the System, authorized an accelerated program, established a new method for apportioning funds among the States, changed the name to the National System of Interstate and Defense Highways, and set the Federal Government's share of project cost at 90 percent.

Title II of the Act - entitled the Highway Revenue Act of 1956 - created the Highway Trust Fund as a dedicated source for the Interstate System.

Revenue from the Federal gas and other motor-vehicle user taxes was credited to the Highway Trust Fund to pay the Federal share of Interstate and all other Federal-aid highway projects. In this way, the Act guaranteed construction of all segments on a "pay-as-you-go" basis, thus satisfying one of President Eisenhower's primary requirements, namely that the program be self-financing without contributing to the Federal budget deficit.

Interstate Design Standards

The Federal-Aid Highway Act of 1956 called for uniform geometric and construction standards for the Interstate System. The standards were developed by the State highway agencies, acting through the American Association of State Highway and Transportation Officials (AASHTO) and adopted by the FHWA. The standards are included in the AASHTO publication A Policy on Design Standards -- Interstate System available from the AASHTO web site. Examples of design standards for the Interstate System include full control of access, design speeds of 50 to 70 miles per hour (depending on type of terrain), a minimum of two travel lanes in each direction, 12-foot lane widths, 10-foot right paved shoulder, and 4-foot left paved shoulder. Initially, the design had to be adequate to meet the traffic volumes expected in 1975. Later, the requirement was changed to a more general 20-year design period to allow for evolution of the System.

Interstate Access

Interstate Access information is available in the following location: https://www.fhwa.dot.gov/programadmin/fraccess.cfm

Interstate Route Numbering

The Interstate route marker is a red, white, and blue shield, carrying the word "Interstate", the State name, and the route number. Officials of AASHTO developed the procedure for numbering the routes. Major Interstate routes are designated by one- or two-digit numbers. Routes with odd numbers run north and south, while even numbered run east and west. For north-south routes, the lowest numbers begin in the west, while the lowest numbered east-west routes are in the south. By this method, Interstate Route 5 (I-5) runs north-south along the west coast, while I-10 lies east-west along the southern border.

In two cases, a major route has two parallel or diverging branches. In those cases, each branch is given the designation of the main route, followed by a letter indicating a cardinal direction of travel (east, west, etc). In Texas, for example, I-35 splits at Hillsboro, with I-35E going through Dallas, while I-35W goes through Fort Worth. The two branches merge at Denton to reform I-35. A similar situation exists along I-35 in the Minneapolis-St. Paul area of Minnesota.

The major route numbers generally traverse urban areas on the path of the major traffic stream. Generally, this major traffic stream will be the shortest and most direct line of travel. Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix.

To prevent duplication within a State, a progression of prefixes is used for the three-digit numbers. For example, if I-80 runs through three cities in a State, circumferential routes around these cities would be numbered as I-280, I-480, and I-680. The same system would be used for spur routes into the three cities, with routes being numbered I-180, I-380, and I-580, respectively. This system is not carried across State lines. As a result, several cities in different States along I-80 may each have circumferential beltways numbered as I-280 or spur routes numbered as I-180.

Interstate Exit Numbers

The States typically use one of two methods of numbering the Interstate interchange exits.

  • The Consecutive numbering system -- Starting at the most westerly or southerly point on each Interstate route, interchanges are numbered consecutively. Thus the first interchange becomes Interchange #1. Each succeeding interchange is numbered consecutively as #2, 3, 4, etc.
  • The Milepost numbering system -- All Interstate routes are mileposted beginning at the most westerly or southerly point. The beginning point is milepost '0'. If the first interchange on the route is located between milepost 4.0 and 5.0, it is numbered as Interchange #4. The next interchange, if located at milepost 8.7, would be numbered as Interchange #8, etc. With this system the motorist can easily determine the location and distance to a desired interchange.
Updated: 02/05/2019
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