Where we are in the world
For those familiar with the highways system in the US, you know that the entire system is a seemingly confusing jumble of interconnected roads (that also always seem to be under construction…). What you might not know is that the US Interstate System set-up is actually more organized than you might think. Before getting into the Interstate System, it might be helpful to define some of the major terms for roads in the US:
- Interstates = federally-funded freeways that form the backbone of road travel between states
- US Highway System = state + local funded roads that are currently important for travel throughout state, but previously used to be vital interstate travel routes before the Interstate system was developed
- State Highways = state-funded roads that primarily serve travel within a single state; these can vary in quality + standards
- County Highways = county-funded road systems that can vary in quality + standards
The US Interstate System is one of the most well-developed and heavily-traveled systems of freeways in the world, and is surprisingly well-organized.
For other maps on road systems, check out posts on Roundabouts in Carmel, Indiana and Rose Park, Missoula!
See the interactive Google Map at the bottom of the post!
Three key facts about this map
Numbering of the US Interstate System follows a systematic pattern
While it might seem that roads are randomly numbered, when looking at a map you can see that there is a method to the madness:
- Starting with I-5 traveling from San Diego to the Washington/Canada border, Interstates traveling North/South are numbered odd
- Starting with I-10 traveling from Santa Monica, CA to Jacksonville, FL, Interstates traveling East/West are numbered even
While there are interstates outside of those numbered in intervals of 5 (for example, I-94, I-29, I-64, etc.) the main routes of the US Interstate System are centered around these major trunk routes. The longest Interstate is I-90 with a distance of more than 3000 miles from Seattle, WA to Boston, MA.
There is apparently no I-50 or I-60 due to overlapping naming systems between the US Interstate System and the older US Highway System.
| Interstate | Direction | Start (approx.) | End (approx.) | Distance (mi.) |
|---|---|---|---|---|
| 5 | North/South | San Diego, CA | Washington/Canada border | 1381 |
| 15 | North/South | San Diego, CA | Montana/Canada border | 1434 |
| 25 | North/South | Las Cruces, NM | Buffalo, WY | 1062 |
| 35 | North/South | Laredo, TX | Duluth, MN | 1568 |
| 45 | North/South | Galveston, TX | Dallas, TX | 285 |
| 55 | North/South | New Orleans, LA | Chicago, IL | 964 |
| 65 | North/South | Mobile, AL | Gary, IN | 877 |
| 75 | North/South | Miami, FL | Sault St. Marie, MI | 1786 |
| 85 | North/South | Montgomery, AL | Petersburg, VA | 666 |
| 95 | North/South | Miami, FL | Maine/Canada border | 1919 |
| 10 | East/West | Santa Monica, CA | Jacksonville, FL | 2460 |
| 20 | East/West | Scroggins Draw, TX | Florence, SC | 1539 |
| 30 | East/West | Fort Worth, TX | Little Rock, AR | 367 |
| 40 | East/West | Barstow, CA | Wilmington, NC | 2557 |
| 70 | East/West | Cove Fort, UT | Baltimore, MD | 2172 |
| 80 | East/West | San Francisco, CA | Teaneck, NJ | 2900 |
| 90 | East/West | Seattle, WA | Boston, MA | 3020 |
Aside from the main routes, the US Interstate System has a method for numbering auxiliary routes
The major trunks are typically numbered with 1- or 2-digits (for example, I-5, I-70, etc.). However, there are many other smaller routes that have 3-digits instead. These 3-digit Interstates are “auxiliary routes” that tend to connect parts of a city directly to one of the major trunk roads.
While there are many exceptions to this, the naming of the road tends to follow these rules:
- First digit:
- If even, the auxiliary Interstate connects to the major interstate in 2 locations (for instance, a loop or a circle around a city)
- If odd, the auxiliary Interstate connects to the major interstate in only 1 location (for instance, a short highway spur from the center of a city out to where the major Interstate road is)
- Second + third digits:
- These are always the major interstate that the auxiliary route is connecting to
Here are a couple of examples:
- I-405 in Los Angeles (one of the most-heavily trafficked portions of the US Interstate System in the country) connects to I-5 in two locations and travels parallel to access different parts of the city of LA
- I-110, also in Los Angeles, connects to I-10, but starts from Long Beach area near the port and connects to I-10 in only one location
The most-congested area of the entire US Interstate System is in the NYC area
Perhaps unsurprisingly, out of the nearly 50,000 miles of Interstate roads, the most-congested portion is on I-95 around the NYC area – specifically right before the George Washington Bridge crossing from New Jersey into Manhattan.
More than 100 million vehicles cross this point over the course of a standard year, and the average speed of the stretch is about 27 miles per hour. Even in non-peak hours, the average speed is only 29.5 miles per hour, showing how consistently congested this specific corridor of the US Interstate system really is.
Additional links and sources
- Article about how highways get their numbers (link, published in 2022)
- FAQs about the US Interstate System from the Federal Highway Administration (link)
- Article about the most congested highways in the US (link, published 2023)
How many of the US Interstates have you travelled on?
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