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

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.

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.

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