FAQs & Film About the Eisenhower System of Interstate and Defense Highways

June 30, 2016

(from an excerpt by David A Pfeiffer, National Archives)

In the summer of 1919, just months after the end of World War I, an expedition of 81 Army vehicles—a truck convoy—set out from Washington, D.C., for a trip across the country to San Francisco.

The convoy's purpose was to road-test various Army vehicles and to see how easy or how difficult it would be to move an entire army across the North American continent. The convoy assumed wartime conditions—damage or destruction to railroad facilities, bridges, tunnels, and the like—and imposed self-sufficiency on itself.

Averaging about 6 miles an hour, or 58 miles a day, the trucks snaked their way from Washington, up to Pennsylvania and into Ohio, then due west across the agricultural Midwest, the Rockies, and into California. Generally, it followed the "Lincoln Highway," later known as U.S. 30, arriving in San Francisco 62 days and 3,251 miles later.

The convoy involved 24 Army officers and 258 enlisted men. One of those officers, a young lieutenant colonel, went along as a Tank Corps observer "partly for a lark and partly to learn," he wrote decades later. "We were not sure it could be accomplished at all. Nothing of the sort had ever been attempted."

The convoy made a lasting impression on the young officer and stoked in him an interest in good roads that would last for decades.

A generation later, during World War II, Dwight D. Eisenhower was still thinking about good roads as Supreme Allied Commander in Europe, where he oversaw the invasion of Western Europe and the defeat of the Nazi army, which was able to move quickly on the autobahns running throughout Germany.

Later, as President of the United States, Eisenhower cited the 1919 convoy and his World War II experiences to persuade Congress to enact the Federal-Aid Highway Act of 1956, creating what is now known as the interstate highway system, which is observing its 50th anniversary this year.

"The old convoy had started me thinking about good, two-lane highways," he wrote years later in his popular memoir, At Ease, "but Germany had made me see the wisdom of broader ribbons across the land."


What was the first Interstate?

On August 2, 1947, the Federal Works Administrator, Major General Philip B. Fleming, approved the first 37,700 miles of the Interstate System recommended by Commissioner of Public Roads Thomas H. MacDonald. Although Congress had not established a program to build the network, the States used Federal-aid primary funds to build many projects in the designated Interstate corridors.

With the approval of the Federal-Aid Highway Act of 1956, the  formal Interstate Construction Program began, with higher design standards, a funding program, and a national commitment.. Two States can claim the first project, depending on how "first" is defined.

The first project to go to construction with Interstate Construction funds under the 1956 Act was in Missouri. The project on U.S. 40 (later designated the I-70 Mark Twain Expressway) in St. Charles County got underway on August 13, 1956. Officials erected a sign stating, "This is the first project in the United States on which actual construction was started under provisions of the new Federal-Aid Highway Act of 1956." 

Kansas had begun a construction project on U.S. 40 (I-70) west of Topeka before the 1956 Act, but awarded the final paving contract under the new legislation. Because this was the first paving under the 1956 Act, Kansas erected a sign claiming, "This is the first project in the United States completed under provisions of the new Federal-Aid Highway Act of 1956."

When did the program end?

It didn't. The program authorized by the Federal-Aid Highway Act of 1956 includes one last piece of Interstate that is likely to be built under the terms of the 1956 Act. It is a connection north of Philadelphia to close the last gap in I-95. The project involves an I-95 interchange with the Pennsylvania Turnpike and an additional bridge over the Delaware River parallel to the existing bridge. Review of the proposed project under the National Environmental Policy Act of 1969 has been completed and detailed design is underway.

In addition, States continue to develop routes outside the 1956 Act program that may be added to the Interstate System under existing legislation or that have been or will be declared "future Interstates" by Federal legislation.

How long is the Interstate System?

Currently, the Interstate System is 46,876 miles long. The Federal-Aid Highway Act of 1956 imposed a statutory limitation on the Interstate mileage that would be built with Interstate Construction funds under the new program (41,000 miles at the time). Later legislation increased the limitation to 43,000 miles, of which a total of 42,795 miles has been used. Separate legislation allows the Federal Highway Administration to approve additional mileage if it meets full Interstate standards and would be a logical addition or connection. Beyond the 42,795 miles, this additional mileage is not "chargeable"—that is, it is not eligible for Interstate Construction funds under the 1956 Act, as amended, although the State may use other Federal-aid funds to help with construction.

What did it cost?

The final estimate of the cost of the Interstate System was issued in 1991. It estimated that the total cost would be $128.9 billion, with a Federal share of $114.3 billion. This estimate covered only the mileage (42,795 miles) built under the Interstate Construction Program. It excluded turnpikes incorporated into the Interstate System within the mileage limitation and the mileage added as a logical addition or connection outside the limitation but financed without Interstate Construction funds.

In all, Federal-aid legislation authorized a total of $119 billion to pay the Federal share of the cost of Interstate construction. (Interstate Construction funds were authorized through Fiscal Year 1996.)  

What is vertical clearance and why did the Department of Defense (DOD) object to the minimum vertical clearance for the Interstate System in the 1950s?

“Vertical clearance” is the distance from the top of the pavement to the bottom of structures crossing over the highway. It is typically at least 1 foot higher than the legal vehicle height, plus an allowance for future resurfacing that could raise the top of the pavement.

Although the Federal-Aid Highway Act of 1956 added the words “and defense” to the name of the Interstate System (now the Dwight D. Eisenhower National System of Interstate and Defense Highways), the primary justification for the network was its civilian benefits, such as economic opportunity, safety, relief of congestion, and evacuation of cities. At the height of the Cold War and with an atomic or hydrogen bomb attack a conceivable possibility, Congress added “and Defense” to the name in recognition of the fact that the Interstate System would benefit the military, too. However, the emphasis on civilian needs was consistent with the position of the Department of War (now Defense, of course) dating to the early 1920s—if we build a road network adequate for civilian needs, it will serve defense needs as well, with some additions to connect with bases or military plants. It would not be possible to justify such an expenditure solely on the basis of military needs.

In developing minimum design standards for the Interstate System, the State highway agencies and the Bureau of Public Roads (BPR) agreed in July 1956 to include a minimum vertical clearance of 14 feet in Policy on Design Standards – Interstate System prepared by the American Association of State Highway Officials (AASHO) and adopted by the BPR for use on Interstate projects. This figure wasn't pulled out of thin air. The DOD had previously indicated, in 1949 and 1955, that a 14-foot vertical clearance was adequate for most military vehicles. However, after the Soviet Union launched its Sputnik satellite in October 1957, the DOD determined that a 17-foot vertical clearance was needed for some larger equipment, such as the Atlas intercontinental ballistic missile, that could not be transported by rail.

This change led to a debate between the DOD and the BPR that that was resolved by a compromise in 1960. On January 27, 1960, the BPR issued instructions to its field offices changing the minimum standard to 16 feet for Interstate highways in rural areas. In urban areas, “application of the 16-foot clearance shall be limited to a single routing where the revised vertical clearance can be developed most economically, even though that single route is indirect.”  All projects under design or construction were to be revised according to the new standard. The first construction project affected by the change was in Michigan, where highway officials using hydraulic jacks lifted the Clear Lake Road overpass on I-94 near Lansing from a clearance of 14-feet, 6-inches to 16-feet, 3-inches.

Who numbered the Interstates?

Following enactment of the Federal-Aid Highway Act of 1956, officials of the U.S. Bureau of Public Roads (BPR) and the American Association of State Highway Officials (AASHO) agreed that AASHO should apply numbers to the Interstate System, as it did to the U.S. numbered highways. In fact, the numbering plan for U.S. numbered highways was the model for the Interstate System—but in mirror image (for example, U.S. 1 is on the East Coast, while I-5 is on the West Coast; U.S. 10 is in the north while I-10 is in the south). With basic guidelines in hand, AASHO's Executive Secretary, A. E. "Alf" Johnson, applied numbers to the Interstate map. His handiwork was approved by AASHO's Route Numbering Subcommittee and Executive Committee and adopted by the BPR in September 1957.

Because the States own the Interstates, cooperation with AASHO (and its successor, the American Association of State Highway and Transportation Officials [AASHTO]) in the numbering of Interstates has continued. However, under Title 23, Code of Federal Regulations, Subsection 470.115(a) ("Approval authority"), the Federal Highway Administration (FHWA) has the ultimate authority to approve/disapprove Interstate route numbers: 

In view of the Federal-State partnership, the State transportation departments submit applications to AASHTO when new numbers are needed or numbering changes are desired. AASHTO submits the applications to the FHWA, which informs AASHTO of its position. AASHTO then acts on the proposal, consistent with the FHWA's action. If disagreement were to occur on a numbering proposal, the FHWA and AASHTO would resolve the issue before AASHTO acted.

Is it true that one out of five miles is straight so airplanes can land on the Interstates?

No. This is a myth that is so widespread that it is difficult to dispel. Usually, the myth says the requirement came from President Dwight D. Eisenhower or the Federal-Aid Highway Act of 1956. However, no legislation, regulation, or policy has ever imposed such a requirement. Airplanes do sometimes land on Interstates in an emergency, but the highways are not designed for that purpose.

Is it true that every so often there has to be a curve in a section of Interstate, to help keep drivers from falling asleep?

Yes and no. Design standards don’t require curves to keep drivers from falling asleep, but that is one reason curves may be included.

Although design standards don’t require curves at specific distances in the alignment of an Interstate highway, curves are introduced for a variety of reasons. The reasons including taking advantage of the terrain along the route; avoiding obstacles or cultural development in the path; and, accommodating environmentally sensitive areas or mitigating impacts on them. A curvilinear alignment also reduces the boredom of driving along extremely long tangent sections (engineer speak for “straight roads”), keeping the driver alert.

Excessive curvature or poor combinations of curvature limit capacity, cause economic losses due to increased travel time and operating costs, and detract from a pleasing appearance. Alignments should be as direct as practical; and consistent with the topography, developed properties, and community values. A flowing line that conforms generally to the natural contours of the land is preferable to an alignment with long tangents slashing through the terrain. Construction scars can be kept to a minimum and natural slopes and growth can be preserved.

The alignment of a proposed highway should be determined by a detailed study of the area through which the road passes. The finished highway, road, or street should be an economical, pleasant, and safe facility on which to travel.

Why do we call Interstate turnpikes “freeways” if they charge tolls?

The term “freeway” refers to how motorists enter and leave the highway, not how the highway was financed. A freeway has full control of access to provide for high levels of safety and efficiency in the movement of large volumes of traffic at high speeds. Freeways have grade separations at all railroads and public crossroads, with interchanges at selected crossroads for access. Traffic can enter on via the interchanges.

The toll turnpikes that have been incorporated into the Interstate System are freeways. They must have full control of access to prevent motorists from using the facility without paying the toll.

Is the left lane for higher speeds?

Although the left lanes of multi-lane freeways are widely viewed as the "high speed lanes" or "the fast lanes," the speed limit applies to all lanes on any given street or highway. The intent is that vehicles going slower than the posted speed limit should stay to the right.

For more FAQs, click here to find histories, memories, audio and video presentations originally collected for the 50th anniversary of the highway system.