Interstate Highway System

The **Dwight D. Eisenhower National System of Interstate and Defense Highways**, commonly known as the **Interstate Highway System**, or the **Eisenhower Interstate System**, is a network of controlled-access highways that forms part of the National Highway System in the United States. The system extends throughout the contiguous United States and has routes in Hawaii, Alaska, and Puerto Rico.

In the 20th century, the United States Congress began funding roadways through the Federal Aid Road Act of 1916, and started an effort to construct a national road grid with the passage of the Federal Aid Highway Act of 1921. In 1926, the United States Numbered Highway System was established, creating the first national road numbering system for cross-country travel. The roads were state-funded and maintained, and there were few national standards for road design. United States Numbered Highways ranged from two-lane country roads to multi-lane freeways. After Dwight D. Eisenhower became president in 1953, his administration developed a proposal for an interstate highway system, eventually resulting in the enactment of the Federal-Aid Highway Act of 1956.

Unlike the earlier United States Numbered Highway System, the Interstates were designed to be an all-freeway system, with nationally unified standards for construction and signage. While some older freeways were adopted into the system, most of the routes were completely new construction, greatly expanding the freeway network in the United States. Particularly in densely populated urban areas, these new freeways were often controversial as their building necessitated the destruction of many older, well-established neighborhoods; as a result of the many freeway revolts during the 1960s and 1970s, several planned Interstates were abandoned or re-routed to avoid urban cores.

Construction of the original Interstate Highway System was proclaimed complete in 1992, despite deviations from the original 1956 plan and several stretches that did not fully conform with federal standards. The construction of the Interstate Highway System cost approximately $114 billion (equivalent to $597 billion in 2022). The system has continued to expand and grow as additional federal funding has provided for new routes to be added, and many future Interstate Highways are currently either being planned or under construction.

Though much of their construction was funded by the federal government, Interstate Highways are owned by the state in which they were built. With few exceptions, all Interstates must meet specific standards, such as having controlled access, physical barriers or median strips between lanes of oncoming traffic, breakdown lanes, avoiding at-grade intersections, no traffic lights and complying with federal traffic sign specifications. Interstate Highways use a numbering scheme in which primary Interstates are assigned one- or two-digit numbers, and shorter routes which branch off of longer ones are assigned three-digit numbers where the last two digits match the parent route. The Interstate Highway System is partially financed through the Highway Trust Fund, which itself is funded by a federal fuel tax. Though federal legislation initially banned the collection of tolls, some Interstate routes are toll roads, either because they were grandfathered into the system or because subsequent legislation has allowed for tolling of Interstates in some cases.

As of 2020, about one-quarter of all vehicle miles driven in the country used the Interstate Highway System, which had a total length of 48,756 miles (78,465 km). In 2020 and 2021, the number of fatalities on the Interstate Highway System amounted to more than 5,000 people annually.