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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.


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 Cost Estimates

A new method of distributing funds among the States was necessitated by the 1956 Act and was based on each State receiving a share of the annual Interstate Construction fund authorization in the same proportion as the cost to complete its System bore to the cost of the System in all States. To achieve a simultaneous completion in all States, the Congress periodically required the BPR, and later FHWA, to develop a new estimate of the cost to complete the System and to serve as the basis for apportionments until the next estimate was prepared.

Between 1958 and 1991, 15 legislatively-mandated estimates were prepared and submitted to Congress. The total cost (State and Federal shares) of the construction of the Interstate System as reflected in these estimates was as follows:

Total Cost of the Interstate System ($Billions)
As Reported to Congress in Interstate Cost Estimates
YearTotal CostFederal Share
1981 Rev119.000106.200
The Revised 1981 Estimate reflects Interstate System completion as redefined by the 1981 Highway Act.
Breakdown of Estimated Cost of the Interstate System ($Billions)
As Reported in the 1991 Interstate Cost Estimate
 Total CostFederal Share
Preliminary Engineering5.619 = 4.5%4.902
Right of Way16.246 = 13.1%14.471
Construction102.391 = 82.4%90.556
Subtotal124.256 = 100.0%109.929
Other Costs *4.6444.371
* Other costs include legislated takedowns from Interstate Construction fund authorizations for State highway planning and research (HPR), metropolitan planning (PL), FHWA administration and research, and contingencies.

Interstate Status

As of October 31, 2002, all but 5.60 miles of the 42,793-mile Interstate System as designated under former Sections 103(e)(1), (2), or (3) of Title 23, and built with Interstate Construction (IC) funds, were completed and open to traffic. The status of these 5.60 miles is shown in the following table:

Sections Not Open to Traffic, but Fully (or Partially) Under Construction / As of October 31, 2002
Massachusetts2.40 milesI-90, Third Harbor Tunnel and approaches
Ohio0.10 milesI-670, Connector near downtown Columbus
Total2.50 miles 
Sections Not Open to Traffic, and Not Under Construction / As of October 31, 2002
Ohio1.60 milesI-670, Columbus Inner Connector
Pennsylvania1.50 milesI-95, Interchange with PA Turnpike
Total3.10 miles 
Note: In addition to the 5.60 miles not open to traffic, there is some remaining Interstate Construction funded work not complete on sections already open to traffic. This work includes major upgradings, added lanes, stage construction, etc. and was reported in the 1991 Interstate Cost Estimate.

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 System Expansion

Mileage Eligible for Interstate Construction Funding

The Federal-Aid Highway Act of 1968 authorized the System's expansion to 42,500 miles. An amendment sponsored by Representatives James Howard and William Cramer designated an additional 200 miles for modification or revision of the basic System. The Federal-Aid Highway Act of 1973 increased the Howard-Cramer mileage to 500 miles. The Surface Transportation Assistance Act of 1978 provided full Interstate Construction funding for all routes designated under previous System adjustments. Another provision of this Act prohibited use of Interstate Construction funds for the construction of any new miles designated after passage of this Act.

"Non-Chargeable" Mileage Additions

Section 16 of the Federal-Aid Highway Act of 1968 provided that the Secretary may designate, as part of the Interstate System, a highway meeting all standards for Interstate highways and that are logical additions or connections to the System. These Interstate additions are not "charged" against the former Section 103(e) mileage limitation for routes eligible for Interstate Construction funding. Codified as Section 139 of Title 23, the provision created no Federal financial responsibility for these Interstate additions. The provisions of this Section 139 of Title 23 have been incorporated in revised Section 103(c)(4) of Title 23.

Section 332(a)(2) of the National Highway System Designation Act of 1995 amended Section 1105 of the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) to designate four high priority corridors as future Interstate routes when the Secretary determines that they meet Interstate design standards and connect to an existing Interstate route. The NHS Act created no Federal financial responsibility for these Interstate additions. Section 1211 of the Transportation Equity Act for the 21st Century modified the description for three of these corridors and designated five additional corridors as future parts of the Interstate System. Interstate route numbers for five of the corridors are also specified by these Acts.

Added Lanes

Interstate Construction funds may not be used for adding lanes to existing Interstate routes, except for a few miles of added lane projects which were included in and approved as part of the 1991 Interstate Cost Estimate. States may use other appropriate classes of Federal-aid funds to add additional capacity to the System.

Added (or Modified) Interchanges

Title 23 USC 111 provides that all agreements between the Secretary and the State highway department for the construction of projects on the Interstate System shall contain a clause providing that the State will not add any points of access to, or exit from, the project in addition to those approved by the Secretary in the plans for such project, without the prior approval of the Secretary. The "Secretary" means the Secretary of the U.S. Department of Transportation and "points of access" means ramps and interchanges. The Secretary has delegated the authority to administer 23 USC 111 to the Federal Highway Administrator. An FHWA policy statement describing the justification and documentation needed for new or modified access points to the existing Interstate System was published in the Federal Register on February 11, 1998. The policy statement can be viewed in Federal Register format at the Government Printing Office web site or it can be viewed in plain text format.

Proposals for additional or modified access points are usually initiated by the State transportation departments but in some cases local governments or private entities initiate the process. In any case, the State highway departments are responsible for developing the information necessary to support an access request and submitting the access request to FHWA for approval.

Interstate System Mileage (as of October 31, 2002)

42,843Includes original mileage authorized by former 23 USC 103(e)(1), Howard-Cramer additions authorized by former 23 USC 103(e)(2), and 1,500 additional miles authorized by former 23 USC 103(e)(3). This mileage includes toll roads and certain free roads incorporated into the Interstate System.
3,546Mileage added to the System under former 23 USC 139(a) and new 23 USC 103(c)(4)(A) without Federal Interstate Construction funding.
337Mileage added to the System under Section 1105(c)(5) of the ISTEA, as amended, without Federal Interstate Construction funding.
46,726Total Miles

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.

Interstate Maintenance (IM) Program

The FHWA first became involved with funding for maintenance activities on the Interstate System as a result of the Federal-Aid Highway Act of 1976 that established the 3R program to fund interstate resurfacing, restoration and rehabilitation. The Federal-Aid Highway Act of 1981 expanded the program by adding a fourth R, reconstruction. The 1991 Intermodal Surface Transportation Efficiency Act (ISTEA) terminated the I-4R program, except for a small discretionary setaside, established a new IM program and a separate NHS program which includes the Interstate System. The IM funds may be used on the Interstate System for 3R work and for reconstruction of bridges, interchanges and overcrossings along existing Interstate routes, but may not be used for the construction of new travel lanes other than high occupancy vehicle lanes or auxiliary lanes.

The 1998 Transportation Equity Act for the 21st Century (TEA-21) expanded eligibility for funding under the IM program to the 4th R, reconstruction. As a result, the addition of new interchanges, new rest areas, new noise walls, etc. became eligible for IM funding. However, IM funding of added lanes, except HOV and auxiliary lanes, are not allowed.

Interstate System Facts

We are often asked questions about the Interstate System which fall into the "Interstate Trivia" category. Some of the more popular trivia is listed below. The data includes mileage that constitutes the 42,794 mile system constructed under the Interstate Construction program, and "non-chargeable" routes and segments added under former 23 USC 139(a), new 23 USC 103(c)(4)(A), and Section 1105(e)(5) of the ISTEA, as amended:

InterstateFrom ToMiles
Longest Interstate Routes:
I-90Seattle, WA to Boston, MA3,020.54 miles
I-80San Francisco, CA to Teaneck, NJ2,899.54 miles
I-40Barstow, CA to Wilmington, NC2,555.40 miles
I-10Los Angeles, CA to Jacksonville, FL2,460.34 miles
I-70Cove Fort, UT to Baltimore, MD2,153.13 miles
Shortest (2-Digit) Interstate Routes:
I-97Annapolis to Baltimore, MD17.62 miles
I-99Bedford to Bald Eagle, PA53.00 miles*
I-73Emery to Greensboro, NC56.70 miles
I-19Nogales to Tucson, AZ63.35 miles
I-66Strasburg, VA to Washington, DC74.80 miles
(* Additional miles of I-73 in NC and I-99 in PA are expected to be built)
East-West Transcontinental Routes:
I-10Los Angeles, CA to Jacksonville, FL2,460.34 miles
I-80San Francisco, CA to Teaneck, NJ2,899.54 miles
I-90Seattle, WA to Boston, MA3,020.54 miles
North-South Transcontinental Routes:
I-5San Diego, CA to Blaine, WA1,381.29 miles
I-15San Diego, CA to Sweetgrass, MT1,433.52 miles
I-35Laredo, TX to Duluth, MN1,568.38 miles
I-55New Orleans, LA to Chicago, IL964.25 miles
I-65Mobile, AL to Gary, IN887.30 miles
I-75Miami, FL to Sault Ste Marie, MI1,786.47 miles
I-95Miami, FL to Houlton, ME1,919.74 miles
States with Most Interstate Mileage:
Texas17 routes3,233.45 miles
California25 routes2,455.74 miles
Illinois23 routes2,169.53 miles
Pennsylvania22 routes1,759.34 miles
New York29 routes1674.73 miles
States with Most Interstate Routes:
New York1,674.73 miles29 routes
California2,455.74 miles25 routes
Illinois2,169.53 miles23 routes
Pennsylvania1,759.34 miles22 routes
Ohio1,572.35 miles21 routes
InterstateFrom ToCost
Most Costly Routes (Eligible for Interstate Construction Funds Based on 1991 Cost Estimate):
I-95Miami, FL to Houlton, ME$8.0 billion
I-90Seattle, WA to Boston, MA$7.5 billion
I-75Miami, FL to Sault Ste Marie, MI$5.1 billion
I-10Los Angeles, CA to Jacksonville, FL$5.0 billion
Interstate Routes Which Traverse the Most States:

Other Interstate Trivia

State Capitals -- All but four State capitals are directly served by the Interstate System. Those not directly served are:

  • Juneau, AK
  • Dover, DE
  • Jefferson City, MO
  • Pierre, SD

Oldest Segments -- The oldest Interstate segments actually predate the establishment of the Interstate system. Early examples include a portion of the Grand Central Parkway in Queens, New York, which was opened to traffic in July 1936 and later was incorporated into the Interstate System as I-278. The Pennsylvania Turnpike between Irwin (southeast of Pittsburgh) and Carlisle (west of Harrisburg) was officially opened in October 1940 and is now designated as I-76 and I-70. Other freeways and toll roads were incorporated into the System rather than build new competing Interstate routes.

Rest Areas -- An exact count of rest areas on the Interstate System is not available. However, a count in 1972 reported 1,214 rest areas in existence. The number still operational today is not expected to differ dramatically from the 1972 figure.

Interchanges -- An exact count of the number of interchanges on the Interstate System is not available. However, a 1978 count found 14,231 interchanges. This number has likely increased somewhat over the intervening years.

Interstate Publications and Links

Interstate Route Log and Finder List, March 2001. This publication includes a listing of all Interstate routes, their lengths, and principal cities served. This can be viewed online by clicking on the title above, or you can obtain a free printed copy by calling Mr. Bing Wong at 202-366-4651. The Internet version is updated periodically, the printed copy is only current as of March 2001.

There is now a site on the Internet for anyone interested in highway history at https://www.fhwa.dot.gov/infrastructure/history.cfm. It brings together a series of articles by FHWA's Richard F. Weingroff from Public Roads magazine about the evolution of highway transportation in the 20th century, along with online only features, and three unique monographs by the FHWA's former "unofficial historian," the late W. Lee Mertz, on the origins and construction of the Interstate System.

The FHWA Internet homepage contains a wealth of information on the Federal-Aid Highway Program and links to homepages maintained by individual States. It's a good source for information on road conditions and active construction work zones.

Interstate System Contacts at FHWA

Updated: 03/30/2018
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000