Is there any rhyme or reason to how the United States interstate highways are numbered? The answer is yes.
Before we break it down, let’s take a step back and look at what is officially known as the Dwight D. Eisenhower System of Interstate and Defense Highways. This massive federal road-building project began in the late 1930s, but only 22 years later did Congress authorize spending. Construction began shortly thereafter and the Federal Aid Highway Act was passed in 1956 which set uniform construction standards for things such as access, speed, the number of lanes, and the width of lanes and shoulders.
Part of the Highway Act included the numbering system. Here is how it works:
Construction of the Interstate Highways System was deemed complete in 1992 for the small cost of approximately $114 billion, but some of the planned routes were canceled and several gaps in the system include stretches that don’t quite conform to federal standards. As for the numbering system, they can still make your head hurt trying to full understand them, but it’s pretty simple when you don’t overthink it.
- Routes with odd numbers run North to South.
- Routes with even numbers run East to West.
- For North/South routes, the lowest numbers are in the West.
- For East/West routes, the lowest numbers are in the South.
- When an interstate reaches a major urban area, they become beltways around a city and carry a three-digit number. These beltways are designated with the number of the main interstate and an even-number prefix. To prevent duplication within a single state, prefixes go up by twos. For example, if I-10 runs through three major metropolises in a state, the beltways would be I-210, I-410, and I-610. The beltway numbering system does not cross state lines, so cities in different states could have a beltway with the same number.
East / West Transcontinental Routes:
- I-10: Runs 2,459.96 miles from Los Angeles, CA to Jacksonville, FL
- I-80: Runs 2,906.77 miles from San Francisco, CA to Teaneck, NJ
- I-90: Runs 3,085.27 miles from Seattle Washington to Boston, MA
North / South Transcontinetnal Routes:
- I-5: Runs 1,382.04 miles from San Diego, CA to Blaine, WA
- I-15: Runs 1,436.89 miles from San Diego, CA to Sweetgrass, MT
- I-35: Runs 1,831.43 miles from Laredo, TX to Duluth, MN
- I-55: Runs 943.69 miles from New Orleans, LA to Chicago, IL
- I-65: Runs 888.08 miles from Mobile, AL to Gary, IN
- I-75: Runs 1,787.49 miles from Miami, FL to Sault Ste Marie, MI
- I-95: Runs 1,892.76 miles from Miami, FL to Houlton, ME