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Quick Guide to the U.S. Interstate Highway System

Updated on August 19, 2011
Collage of interstate highway signs
Collage of interstate highway signs | Source

The Basics

Most have experienced the horror of getting lost while driving, but making use of the parameters outlined by the U.S. Interstate Highway System can provide some basis for location when stuck in that all-too-familar jam. In addition, navigation for longer trips can be simplified by using signage and landmarks when there is no functioning GPS present.

The Interstate Highway System exists as a vital staple of automobile travel throughout most of the contiguous United States. In some cases, gorgeous scenery was sacrificed to make way for shorter and more efficient travel times. Nonetheless, the ins and outs of its organization prove fascinating. Construction and planning still continue today as the system is only fifty-five years removed from its official inception in the mid-1950s. Despite its relative infancy, it has become almost natural to envision an 18th- and 19th-century America densely packed with the crisscross and zigzag of main thoroughfares and auxiliary routes coordinated into an elaborate system of connection. Yet, many still remain unaware of how its numbering system works and the distinctive intricacies it utilizes. Here is a short list of helpful facts when traveling:

  1. Markers. Main interstate highways are signified by the standard "I-" (interstate) and two-digit numbers that range from 4 to 99. These blue and red shield-shaped signs appear frequently along both sides of the freeway. In some cases, the name of the state where the sign is located will appear in small font below the word "Interstate."
  2. Major interstates. Interstates that cover longer treks and make use of several auxiliary routes are marked by numbers that end in "5" or "0." For instance, I-95 nearly runs along the entire eastern seaboard from Maine to Florida.
  3. Even vs. odd. Even-numbered highways follow a true east-west or horizontal direction, while odd-numbered highways follow a true north-south or vertical direction. Naturally, some of these routes will run in auxiliary directions (NE or SW, for example) and may run perpendicular to their true course. Overall, though, major highway numbers strictly abide by these guidelines.
  4. Major city signs. Each interstate highway features a series of green "checkpoint" signs that mark which major city the traveler will encounter next. These large overhead signs contain the name of the next major city under the specific interstate's logo. One example is I-90, which has markers for Seattle, Chicago, Boston, and other cities along the way.
  5. Concurrent routes. In some cases, interstate highways will run concurrently with one another and/or minor routes. For instance, I-80 runs concurrently with I-90 for a portion of the mideastern United States, while I-95 runs concurrently with U.S. Route 1 on several occasions. Signs for each merged route will appear together on the side of the highway.
  6. Auxiliary routes. Three-digit interstate highways represent auxiliary routes that supplement major highways. These will feature a single digit sandwiched by the standard "I-" and a two-digit interstate number. The first digit can range from 1 to 9 and the final two digits are determined by the major highway the auxiliary route supplements (I-395 and I-287, for instance). They are generally used to bypass or to provide an alternate route through major cities. Wherever necessary, auxiliary routes shift between traveling true north-south and true east-west. Many major interstates offer multiple auxiliary routes.

Sign for I-80 in Nevada that features the name of the state on the sign
Sign for I-80 in Nevada that features the name of the state on the sign | Source
Checkpoint sign marking I-90 & I-94, which run concurrently in this location
Checkpoint sign marking I-90 & I-94, which run concurrently in this location | Source

Did You Know?

The following are some intriguing facts about the interstate highway system:

  • Some routes are actually intrastate highways, despite their names. The "interstate" portion of each name is a reference to funding, not to actual areas served. Examples include I-4 in Florida and I-87 in New York.
  • I-80 and I-90 both almost traverse the entire width of the United States! A person traveling on I-80 in New Jersey would technically be on the same road as a family about to enter San Francisco via I-80. Similarly, travelers near Boston and Seattle make use of the same road when traveling on I-90.
  • I-90 in New York is the only interstate highway to make use of all nine-digit auxiliary route options in one state (I-190, I-290, I-390...I-990).
  • Mileage in each state for a specific highway begins at the western or southernmost point of the highway in that state and increases until reaching the eastern or northernmost point of the highway in that state. This also applies to exit numbers, whether they are listed sequentially or correspond to mileage traveled in state. Thus, if the exit numbers are decreasing, you are traveling west on an even-numbered highway or south on an odd-numbered highway!
  • Paralleling exit numbers and mileage markers, the lower even-numbered highways start in the southern United States and increase northward (I-4 runs through Florida, while I-96 runs through Michigan). Likewise, lower odd-numbered highways start in the western U.S. and increase eastward (I-5 runs along the western seaboard, while I-95 runs along the eastern seaboard).
  • Four interstate highways have eastern and western versions: I-76, Colorado to Nebraska and Ohio to New Jersey; I-84, Oregon to Utah and Pennsylvania to Massachusetts; I-86, in Idaho and from Pennsylvania to New York; I-88, in Illinois and also in New York.

In addition to these interesting facts, there is a wealth of information available online.

U.S. Interstate Highway Map
U.S. Interstate Highway Map | Source


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