Interstate Highway System
The Dwight D. Eisenhower National System of Interstate and Defense Highways (often called the Interstate Highway System, Interstate or I-), is a group of roads in the United States. They are grade separated, which means drivers must use a specially made group of ramps, called an interchange or exit, in order to get on or off the Interstate. Because of this, people are allowed to go very fast, sometimes up to 80 miles per hour (130 km/hr).
The standards setting body which publishes specifications, quality control protocols and guidelines for the highway design and construction throughout the United States is AASHTO.
Interstates have a special way to figure out their numbers. The main Interstates have numbers with 1 or 2 digits. Interstates that run east/west are even numbers (2, 4, 6, 8, or 0), and north/south Interstates are odd numbers (1, 3, 5, 7, or 9). For east/west Interstates, the small numbers are in the south and the big numbers are in the north. For north/south Interstates, the small numbers are in the west and the big numbers are in the east. The important east/west Interstates have numbers ending in "0" and the important north/south Interstates have numbers ending in "5".
- Interstate 10 is an important east/west Interstate in the south part of the United States from Los Angeles, California to Jacksonville, Florida
- Interstate 90 is an important east/west Interstate in the north part of the United States from Seattle, Washington to Boston, Massachusetts
- Interstate 5 is an important north/south Interstate along the west end of the United States from San Diego, California to Washington
- Interstate 95 is an important north/south Interstate along the east end of the United States from Miami, Florida to Maine
Three-digit interstates [change]
Some cities have short freeways that get a three-digit number. That means that there is an extra number put ahead of the number of the Interstate. This number is even (2, 4, 6, or 8) if the Interstate goes around a city and connects with another Interstate, and odd (1, 3, 5, 7, or 9) if an Interstate goes into a city and ends at a road that is not an Interstate Highway. Numbers can be used again in different states. Three-digit Interstates usually go to the Interstate in their last two numbers. For example, Interstate 195 goes to Interstate 95.
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