The Origin of the Interstate System
Other major factors
http://en.wikipedia.org/wiki/Interstate_Highway_System#Planning
“The United States government’s efforts at constructing a national network of highways began on an ad hoc basis with the passage of the Federal Aid Road Act of 1916, which provided for $75 million over a five-year period for matching funds to the states for the construction and improvement of highways.[5] The nation’s revenue needs associated with World War I prevented any significant implementation of this policy, which expired in 1921.
As the landmark 1916 law expired, new legislation was passed—the Federal Aid Highway Act of 1921 (Phipps Act). This new road construction initiative once again provided for federal matching funds for road construction and improvement, $75 million allocated annually.[5] Moreover, this new legislation for the first time sought to target these funds to the construction of a national road grid of interconnected “primary highways” setting up cooperation among the various state highway planning boards.[5]
The Bureau of Public Roads asked the Army to provide a list of roads it considered necessary for national defense.[6] In 1922 General John J. Pershing, former head of the American Expeditionary Force in Europe during the war, complied by submitting a detailed network of 200,000 miles of interconnected primary highways—the so-called Pershing Map.[5]