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The National Highway system of the United States refers to a network of highways designed for traffic moving at a high speed with all entry and exit points carefully regulated. The system has been in operation since its formation on June 29, 1956. Each Interstate Highway is assigned a route number that can be either one or two digits. On the other hand, Auxiliary or Three-digit Interstate Highways have three letters as the route number. Generally, highways with odd numbers go from south to north while those with even numbers run in a west to east direction. In the case of the auxiliary system, each highway has a special number that indicates its parent highway. The longest Interstate Highway in the US is the I-90.

Longest Interstate Highways in the United States

Interstate 90 (I-90)

This transcontinental highway, which was constructed back in 1956, moves in an east to west direction. The western terminus is situated in Seattle, Washington while the eastern terminus is located in Boston, Massachusetts. The massive highway passes through 13 states which are Washington, Idaho, Montana, Wyoming, South Dakota, Minnesota, Wisconsin, Illinois, Indiana, Ohio, Pennsylvania, New York, and Massachusetts. During its construction, this highway made use of roads that were already in existence. For this reason, some parts are of lower quality than others. Parts of older routes that are part of the I-90 include the New York State Thruway, Indiana Toll Road, and others.

Interstate 80 (I-80)

The I-80 is also a transcontinental highway that was constructed back in 1956. The road, which goes from east to west, starts in the downtown parts of San Francisco in California and ends at Teaneck, New Jersey. This highway crosses through 11 states which are California, Nevada, Utah, Wyoming, Nebraska, Iowa, Illinois, Indiana, Ohio, Pennsylvania, and New Jersey. The last section was completed and opened back in 1986.

Interstate 40 (I-40)

Unlike the two highways above, this one was constructed a year later in 1957. The highway goes from east to west from Barstow, California to Wilmington, North Carolina. The highway passes through eight states which are California, Arizona, New Mexico, Texas, Oklahoma, Arkansas, Tennessee, and North Carolina. The last section of this road, which acts as a connection between Wilmington and Raleigh, was finished in the later stages of the 1980s.

Interstate 10 (I-10)

This highway was constructed back in 1957 although it was not completed until 1990. The road goes from east to west from Santa Monica, California to Jacksonville, Florida. The highway goes through eight states which are California, Arizona, New Mexico, Texas, Louisiana, Mississippi, Alabama, and Florida. Texas contains about one-third of the highways total length. 

Newer Highways

In order to improve the system, the US government has been coming up with newer highways that are not on the list. The newest systems are the I-14, I-11, and I-87, which were all constructed in 2017. However, the new ones are considerably shorter than the old ones.