An Interstate highway is a type of major road or freeway.
The Dwight D. Eisenhower System of Interstates and National Defense Highway System, commonly called the interstate highway system, is a network in the United States of interstate highways or simply interstates.
While the name implies highways that cross U.S. state lines, many interstates don't. Rather, it is the system of interstates that connects states. There are interstate highways in Hawaii, funded in the same way as in the other states, but entirely within the islands of Hawaii. Similarly, both Alaska and Puerto Rico have public roads that receive funding from the interstate program, though these routes are not signed as interstate highways.
The heavy role of the federal government in road financing has allowed it to pass laws
in areas outside of the direct power of the government. By threatening to withhold highway funds, the federal government has been able to force state legislatures to pass laws which increase the drinking age to 21 and for a number of years, reduce the maximum speed limit to 55 miles per hour.
Traffic lights are limited to toll booths (and toll booths are limited to grandfathereded roads and bridges), draw bridges, and ramp meters (metered flow control) for lane merging during rush hours.
One potential civil defense use of the Interstate highway system is for the emergency evacuation of cities in the event of a potential nuclear war. Obviously, the economic disruption caused would be considerable. An option for maximizing throughput is to reverse the flow of traffic on one side so that all lanes become outbound lanes.
A widespread but false urban legend states that one out of every five miles of the Interstate highway system must be built straight and flat, so as to be usable by aircraft during times of war.[1]
Three-digit route numbers, consisting of a single digit prefixed to the number of a primary Interstate highway, are used to designate highway extensions, spurs (odd prefixes), and bypasses (even prefixes) that connect to the main highway within an urban area. For example, there are many extensions to I-80 in the San Francisco Bay Area: I-280 connects San Francisco and San Jose; I-380, I-580, I-680, I-780, I-880, I-980 are also major highways. (I-480 was also an extension before it was demolished following local popular opposition). Three-digit route numbers may be repeated in different states for different roads.
Interstate 238 near Oakland, California is one of two exceptions to the numbering scheme, as no Interstate 38 exists (this number exists because Interstate 238 replaced a segment of California Highway 238 and changing the number would have split the California Highway in two segments. The original number was retained to keep the California Highway contiguously numbered). The other exception is I-99 in Pennsylvania which was written into law as I-99 by Pennsylvania Congressman Bud Shuster, as it is west of several Interstates that are numerically less than 99.
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.
An Interstate highway is a type of major 
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