Interstate Highways North-South Routes East West Routes Sequential Table 1958 Map Personal Proposals

Dwight D. Eisenhower National System
of Interstate and Defense Highways

The origins of a national network of highways came with the passage of the Federal Aid Road Act of 1916. This act provided $75 million over a five year period of matching funds to the states for the construction and improvement of highways. World War I would eventually prevent most of these funds from being spent by the time the act expired in 1921. It was replaced by the Federal Aid Highway Act of 1921, also known as the Phipps Act. The Phipps Act once again provided federal funds, in the amount of $75 million annually, and also attempted to target these funds for the construction of a national road grid of interconnected "primary highways." This would also require the various state highway planning boards to cooperate among themselves as to routings.

The first proposed national network of highways came in December 1918. E. J. Mehren, a civil engineer and the editor of Engineering News-Record, presented his "A Suggested National Highway Policy and Plan" during a gathering of the State Highway Officials and Highway Industries Association at the Congress Hotel in Chicago. In the plan, Mehren proposed a 50,000-mile system, consisting of five east-west routes and 10 north–south routes, citing both commercial and military transport benefits. In 1922, General John J. Pershing, former head of the American Expeditionary Force in Europe during the war, submitted a detailed network of 20,000 miles of interconnected primary highways—the so-called Pershing Map. This was in response to a request from the Bureau of Public Roads for a list of roads the Army considered necessary for national defense.

Meanwhile in Europe, a new kind of highway was being designed, a controlled-access superhighway. Italy was the first country in the world to build motorways reserved for motor vehicles. The Milano-Laghi motorway, connecting Milan to Varese, was devised by Piero Puricelli, a civil engineer and entrepreneur. He received the first authorization to build a public-utility fast road in 1921, and completed the construction (one lane each direction) between 1924 and 1926. In Germany between 1929 and 1932, a highway some 12 miles long that also resembled modern interstates except for the lack of a median strip was built between Cologne and Bonn using unemployed labor. That road was the basis for Germany's Autobahns, many of which were first constucted in the 1930s during the Nazi era. Those superhighways would inspire the contruction of Connecticut's Merritt Parkway and the Pennsylvania Turnpike, which opened in 1938 and 1940, respectively.

After World War II and thoughout the 1950s, inspired by the Pennsylvania Turnpike and indicative of the kinds of highways Americans wanted, a growing network of toll superhighways began to spring up. These would include the New York Thruway, Ohio Turnpike, Indiana Toll Road, New Jersey Turnpike, Massachusetts Turnpike and Florida's Turnpike, among others. These were built on a state by state basis and funded largely by the collection of tolls. Other states such as California began constructing a vast system of free superhighways (freeways) along many of their existing U. S. Routes. The latter had a powerful advocate in President Dwight D. Eisenhower. As a young Army officer, he had taken part in a two-month long, cross-country convoy along the Lincoln Highway in 1919. He later credited the experience with showing him the importance of good two-lane highways. His later observations of the German Autobahn network during World War II led him to believe in the need for a nationwide network of four-lane access-controlled highways.

As President, Eisenhower would propose in 1954 and gain passage of the Federal Aid Highway Act of 1956 on June 29, 1956. This act would establish the Interstate Highway System, which was intended to supplement, not replace, the U. S. Highway System. It was purposely laid out as a grid in the opposite direction of the U. S. Highway grid. North-south routes would be given odd numbers, with routes ending in 5 being primary, while east-west routes were given even ones, with routes ending in zero being primary. While the U. S. Route numbers ascended to the west and south (U. S. 1 in the east and U. S. 2 in the north), Interstate numbers ascended to the east and north (I-5 in the west and I-10 in the south). This was so that no Interstate and U. S. Route would share the same number in the same state, though it was initially violated by I-24 and U. S. 24 both being in Illinois, as well as I-40 and U. S. 40 and I-80 and U. S. 80 being in California. It has since been further violated by I-41, I-69, I-74. That also explains why no I-50 or I-60 have ever been assigned, given that they would more than likely share a corridor with U. S. 50 or U. S. 60.

The primary interstate grid is filled out with all one and two-digit numbers. In the beginning, split numbers, i. e. I-80N or I-80S rather than I-80, were fairly common, however most were eliminated in 1963. I-35 has two splits, in Dallas-Fort Worth and Minneapolis-St Paul. Furthermore, I-69 has a congressionally mandated split into I-69E, I-69C and I-69W in order to facilitate three border crossings in south Texas. Three-digit numbers are reserved for spurs and beltway loop routes. Three-digit interstates that begin with an odd number are considered to be spurs, while those that begin with an even number are considered loops, though there are some exceptions. For example, the beltway surrounding Washington D. C. is I-495 and serves as a complete loop around the city. I-395 is a spur route from the south into downtown and I-295 functions as an urban loop within Washington D. C. Exceptions to this rule include I-376 and I-476 in Pennsylvania, though the former is an exception due to a later routing change. The latter was specifically built as a northeastern extension to the Pennsylvania Turnpike (I-76) and assigned three-digit spur number.

The Interstate System was intended to serve all 48 contiguous states, though it does also serve Alaska, Hawaii and Puerto Rico through special prefixed routes (A-x in Alaska, H-x in Hawaii and PR-x in Puerto Rico, where the route number is x). The initial cost estimate for the system was $25 billion over 12 years; the final cost was $114 billion and construction took 35 years. The final original section of the system was completed on October 14, 1992, when I-70 was completed through Glenwood Canyon in Colorado. The system has since been expanded with new routes being added to it, including I-2 in Texas, added in 2013, I-41 in Wisconsin, added in 2015 and a Canada to Mexico extension of I-69 that is ongoing. Routes proposed and awaiting construction include the I-11 Canamex Corridor and I-14, also known as the 14th Amendment Highway.



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