Guide to Understanding the U.S. Highway Numbering System

For those traveling on the highways and byways today, it’s easy to get used to all the electronic gadgets such as GPS, cell phones, built-in navigational systems, and services that provide directions and help get you to your destination. However, it still doesn’t hurt for you to know and understand the road signs while traveling.

Interstate Highway route designations are usually found on the familiar blue shield with white letters and a red crest across the top. The highways are often designated I-## or Interstate ##, and these highways are usually limited access, divided highways. They are usually referred to as freeways or expressways, especially in urban areas, although a few of the interstate routes may have tolls on certain portions of the road.

The Interstate Highways are numbered in a particular system, with the odd-numbered routes running from North to south and the even-numbered routes running from east to west.

For example, I-5 runs from the Washington-Canadian border all the way down to the Mexican border near San Diego. The pattern for the numbering system also begins with the lowest-numbered odd routes starting on the west coast and going to the highest-numbered routes on the east. So, I-5 runs through the Pacific Coast states of Washington, Oregon, and California. As you work your way across the country to the East Coast, I-95 runs from Maine to Florida.

The east-west Interstate routes have their lowest numbered routes starting in the southern United States and going to the highest numbered routes in the North.

I-4 is the lowest route and runs only across the central part of Florida. I-10 is the lowest-numbered major route and runs from Florida to California. I-96 is the highest-numbered route and runs from Detroit to Muskegon, Michigan. I-90 is the highest-numbered major route that extends from Boston to Seattle.

Interstate 96

There are some exceptions to these general rules such as I-99 in Pennsylvania which is numerically higher than I-95 but lies west of it and was the nearest odd 2-digit number available. I-82 in Washington & Oregon lies fully north of I-84; but its numbering is due to I-84 previously having the designation of I-80N. Some other exceptions are I-69 in Michigan, I-76 in Nebraska, and I-64 in Virginia.

The Auxiliary Interstate routes are the three-digit interstate routes that are either loop routes, spurs, or other alternate routes that are extensions of the primary or parent interstate route. Typically, for loop routes, which start from the primary route and return to the same primary route, the first digit is even.

For example, I-495 loops around Washington, DC, and is tied to the I-95 primary route. A spur route is one that extends from the primary route and does not return to an interstate route. I-395 is a spur route that begins from I-95 in Virginia and leads to downtown Washington, DC. Another example of a loop route is I-285 around Atlanta, which is tied to the primary route of I-85.

Many cities and states have spur or loop routes, and the numbers may be used in different locations, except that no state has two auxiliary highways with the same number.

For example, I-295 is used as a loop around Jacksonville, Florida; I-295 is a loop around Philadelphia on the New Jersey side of the metro area; I-295 is a route in the Bronx and Queens; I-295 loops around Richmond and Petersburg, Virginia; I-295 loops around Providence, Rhode Island; and I-295 is a route through Portland, Maine.

In Alaska, Hawaii, and Puerto Rico, the federal highway system (it’s hard to call them interstates since they don’t run between states) uses rules based primarily on their funding and creation.

Two-Digit Routes

The U.S. Highways are numbered in a particular system, with the two-digit odd-numbered routes generally running from North to south and the two-digit even-numbered routes running from east to west. The pattern for the numbering system also begins with the lowest-numbered odd routes starting on the East Coast and going to the highest-numbered routes on the West Coast. For example, U.S. Highway 1 extends from Fort Kent, Maine, to Key West, Florida, on the east coast. U.S. Highway 101, which incidentally is considered a two-digit route, extends from Port Angeles, Washington, to Los Angeles on the west coast. U.S. 2 runs from Houlton, Maine, to Bonners Ferry, Idaho, across the northern United States. U.S. 98 runs from Bartow, Florida, to across the southern U.S. The U.S. Highway system numbering system, which increases from east to west and North to south, is the reverse of the Interstate system to minimize confusion in the numbering system.

Initially, the highways that ended in zero or one were considered the main routes, and many of these extended from one coast to the other or from the northern boundaries of the country to the southern boundaries. With the advent of the Interstate system, which often parallels the U.S. Highways, many of the U.S. Highway routes have been truncated, decommissioned, or extended to the point that they may not adhere to the original layouts when the system was initially established. For example, the legendary Route 66 has been decommissioned and replaced by the Interstate Highway network.

Three Digit Routes

Three-digit numbered routes are spurs of the parent routes and may branch off from the parent or run near the parent. For example, U.S. 231 begins as a spur of U.S. 31 and runs generally near U.S. 31 from St. John, Indiana, near Gary to Panama City, Florida. Some of these may not follow the original pattern of the highway system due to the changes over the years and the altering of the two-digit numbered routes. The first digit of the three-digit route increases along the parent route to follow the east-west and north-south numbering system. For example, U.S. 111 in Maryland, U.S. 211 in Virginia, U.S. 311 in North Carolina, and U.S. 411 in Tennessee, which are spur routes of the parent route U.S. 11, are numbered higher as they go from east to west.

Bannered Routes

Bannered routes are auxiliary highways that are loops, spurs, or alternate routes and have a banner above or below their number on the signage to designate the route. Common banners are ALT (Alternate), BUS (Business), BYP (Bypass), and TRUCK.

Divided Routes

Divided routes are evenly split routes that use the designation of E (east), W (west), N (North), or S (south) to designate the alternate routes. This is not the same as divided highways, which refer to the dividing of the lanes on a specific route. For example, U.S. 9W is a split route between Fort Lee, NJ, and Albany, NY. These divided routes are becoming less common as new designations are not being approved and older ones are being eliminated.

As noted earlier, there are exceptions to these rules and as additional interstate highways are built, it will make it more difficult to conform to the numbering system as available numbers to fit the pattern become limited.

Photo of author
About the author
After a few years of writing for small local newspapers and freelancing for numerous national publications, Shaun took his skills to the Internet. Shaun's work has appeared on various sites and he is ready to tackle new topics and learn new things in the world of journalism.