List of Interstate Highways
There are 70 primary Interstate Highways in the Interstate Highway System, a network of controlled-access freeways in the United States. They are assigned one- or two-digit route numbers, whereas their associated "auxiliary" Interstate Highways receive three-digit route numbers. Typically, odd-numbered Interstates run south-north, with lower numbers in the west and higher numbers in the east; even-numbered Interstates run west-east, with lower numbers in the south and higher numbers in the north. Highways whose route numbers are divisible by "5" usually represent major coast-to-coast or border-to-border routes (ex. I-10 travels from Santa Monica, California, to Jacksonville, Florida, traveling from the Pacific to Atlantic oceans). Additionally, auxiliary highways have their numbering system where a different number prefixes the number of its parent highway.
Transportation in the United States
The advent of the automobile signaled the end of railroads as the predominant transportation for people and began an era of mobility in the United States that added greatly to its economic output. The early 20th century Lincoln Highway and other auto trails gave way in the 1920s to an early national highway system making the automobile the preferred mode of travel for most Americans. Interurban rail service declined, followed by trolley cars due in part to the advent of motorized buses and the lack of dedicated rights-of-way but also by deliberate efforts to dismantle urban rail infrastructure (see Great American streetcar scandal). The scarcity of industrial materials during World War II slowed the growth of the automobile, briefly reemphasizing much of the nation's declining rail network. In the 1950s, the United States renewed building a network of high-capacity, high-speed highways to link its vast territory. The most important element is the Interstate Highway system, first commissioned in the 1950s by President Dwight D. Eisenhower and modeled after the Italian autostrada and the German Autobahn system.