The U.S. Interstate System: Why Is It Numbered The Way It Is?


Ever wondered why the American Interstate highways are numbered the way they are? Here's the answer!

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Interstate vs Highway Interstate Highway System
Interstate vs Highway
Interstate Highway System

GPS systems have made it easier than ever to travel, giving you the power to simply enter the destination you want to go to, with the best route to get there given to you instantly. You can start your journey to anywhere in America without you even having to look at signboards or a map, these days. But you would undoubtedly have noticed the numbered shields and boards that identify the very many intersecting highways as you drive. It's always a good idea to know where you are and on which road you find yourself in case of emergencies, and relying solely on GPS is a bad driving habit you should avoid. It pays to have a basic understanding of the network of interstates, so if ever you've wondered whether or not there's any logic to their numbering, you've come to the right place.

Origins of the Interstate Highway System

The USA has one of the most extensive networks of interstate highways in the world, connecting states throughout the country, with routes into Alaska, Puerto Rico, and Hawaii. While the first car with a combustion engine arrived on the scene in the late 1800s, formalized roads with speed limits and regulations were still a century away. But, in order to know how The National Highway System works, it's good to delve into a little US history.

The US federal government began constructing national roads in the 1920s, with the United States Numbered Highway System created in 1926 back when there were no standards for road design. There wouldn't be any guidance for development and signage until the Federal Aid Highway Act of 1956, which also saw the construction of the highway system kick into gear with rules and regulations for how these would be named and numbered. The original Interstate Highway System was completed in 1992 at a cost in excess of $535 billion by modern standards, although it continues to expand.

Before you pack up your Cadillac Escalade on that cross-country road trip, have a look at the standard and numbering system for US interstates.

Interstate Numbering System

Standards And Numbering For Interstate Routes

In 1926, the United States Numbered Highway System was established as the first official mapping and numbering framework of America's national roads and highways, but it was only with the Act developed in 1956 that a proper system, guided by standards and regulations, was implemented across the country.

The American Association of State Highway and Transportation Officials (AASHTO) sets out various standards that interstates need to adhere to, and include the following:

  • Controlled access is an absolute necessity; traffic lights, toll booths, and ramp meters are allowed
  • Speed limits: These may be determined by the individual states, but federal law provides the foundation. Typically, Northeastern and coastal states have lower speed limits while inland states have higher limits.
  • At-grade intersections must be avoided
  • Federal traffic sign specifications must be complied with
  • Exceptions to these standards can only be granted by a waiver from the Federal Highway Administration

Today, the names and numbers of roads, highways, and interstates in the US are allocated by AASHTO as developed in 1957. When it comes to the interstate numbering system, in particular, these are the guidelines (although some exceptions to the rule exist):

  • Primary interstates get one- or two-digit numbers
  • Shorter routes, including loops, short connecting roads, and spurs, get three digits, with the last two digits matching the parent route
  • Primary routes running from East-West are assigned even numbers
  • For North-South routes, odd numbers are assigned
  • Any route number divisible by 5 is seen as a major artery that carries traffic over long distances
  • Concurrency, or overlap, is allowed where two routes can share the same roadway
Highway Numbering System


What is the longest interstate highway in the US?

The historic Route 66 covered a total of 2,448 miles, and while that sounds like a blast with the top down in your convertible Ford Mustang, it's not the longest highway in the US. Spanning 3,085.27 miles across the country, the I-90; which connects Seattle, Washington to Boston, Massachusetts, is the longest interstate in America. The total length of the US Interstate system is said to be 48,756 miles, however - a fun fact to remember!

What 4 state capitals are not served by the interstate system?

There are four state capitals that are directly connected by the interstate system, including Juneau in Alaska, Dover in Delaware, Jefferson City in Missouri, and Pierre in South Dakota.

Who oversees the highway system in the USA?

Interstate highways are technically owned by the state in which the section of road is built, and while each can have a say about road rules and regulations, federal laws are fundamental to what is, and what isn't, allowed.

What’s the difference: interstate vs highway?

The main difference between an interstate and a highway is that interstates are restricted access roadways that traverse across state boundaries to connect states, while highways are controlled access or limited access roadways that typically traverse through cities. The highway numbering system revolves around the interstate with which a specific highway connects.

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