Interstate Highway System

The Interstate highway system is a network of controlled access superhighways in the United States. It was started under President Dwight Eisenhower in 1956. Although construction is ongoing, it was regarded as complete in 1990. The system was modelled after the German autobahn system and was begun after the completion of the largely at-grade United States highway system.

Interstate highways do not have at-grade intersections. All intersections use overpasses, underpasses, and cloverleafs. All access to the highway is via access ramps that do not interfere with the flow of traffic. Traffic lights are limited to toll booths (and toll booths are limited to grandfathered roads and bridges), and metered flow control for lane merging during rush hours. On the east coast of the US, sections of some interstate highways planned prior to 1956 are operated as toll roads, and are often called turnpikes. Vehicles are expected to travel above 45 mph under normal conditions. Speed limits vary according to location, ranging from 55 mph to 75 mph. Before the oil embargo crisis in the 1970s, some states posted no speed limit on the highway. In 1974, Congress imposed a nationwide 55 mph speed limit by threatening to withhold highway funds from states that did not adopt this limit. This limit was unpopular, especially in Western states, and the restriction on speed limit was lifted in the late-1980s.

On maps and the road, the highway is indicated by a number on a red, white and blue sign in a shape of a shield. By definition, these highways cross multiple states. Much of the construction and maintenance cost is funded through user fees, primarily the gas tax, collected by states and the federal government.

Some old local highways may be renamed when included in the federal system. For example, part of California highway 17 connecting Oakland and San Jose was renamed as I-880 in the mid 1980s. Part of the original California highway 17 still connects San Jose and Santa Cruz.

There are two numbering schemes for the interstate highway system. All even-numbered Interstate highways go generally east-west, and odd-numbered Interstate highways go generally north-south. (However, in some places two or more interstate highways run along the same physical road, and such a road may be "east" for one number and "north" for the other.) For example, I-80 connects San Francisco in the west and Ridgefield, New Jersey in the east. I-5 goes from Washington state in the north to Mexico in the south along the west coast.

The second numbering scheme is for the highway extensions that connect to the main highway within an urban area. A numeric prefix is added to the main highway number. For example, there are many extensions to I-80 highway in the San Francisco Bay Area: I-280 connects San Francisco and San Jose; I-380, I-580, I-680, I-880, I-980 are also major highways. I-480 was also an extension before it was demolished following the earthquake of 1987. Highway extensions are local to the urban area that the main highway goes through. For example, there can be other I-280 extensions in other parts of the country.