Dwight D. Eisenhower enacted the Federal Aid Highway Act of 1956, popularly known as the National Interstate and Defense Highways, when he signed it on June 29, 1956. With an original authorization of 25 billion dollars for the construction of 41,000 miles of the Interstate Highway System supposedly over a 10-year period, it became the largest public works project in American history at the time.
The money for the Interstate Highway and Defense Highways was handled in a Highway Trust Fund that paid for 90 percent of highway construction costs with the states required to pay the remaining 10 percent. It was expected that the money would be generated through new taxes on fuel, automobiles, trucks, and tires.
Eisenhower's support of the Federal-Aid Highway Act of 1956 can be directly attributed to his experiences in 1919 as a participant in the U.S. Army's first Transcontinental Motor Convoy on the historic Lincoln Highway – the first road across America. The highly publicized convoy intended, in part, to underscore the need for better main highways and continued federal aid. The convoy left the Ellipse south of the White House in Washington D.C. on July 7, 1919 and headed for Gettysburg, PA before turning west toward San Francisco.
Along the way bridges cracked, vehicles became stuck in mud, and equipment broke, but communities across the country greeted the convoy as it passed. For Eisenhower, the experience convinced him to support construction of the Interstate System when he became President.
Eisenhower argued for the highways for the purpose of national defense. In the event of a ground invasion by a foreign power, the U.S. Army would need good highways to transport troops across the country efficiently. Following completion of the highways, the cross-country journey that took the convoy two months in 1919 was cut down to two weeks.
He announced his "Grand Plan" for highways in 1954, which led to the 1956 legislative breakthrough that created the Highway Trust Fund to accelerate construction of the Interstate System.
• Wikipedia, Federal-Aid Highway Act