How the U.S. Interstate Highway System Works

You will never look at a road trip the same way again.

How U.S. Interstate Highway System Works

Moderate traffic on a U.S. highway — Millions of cars travel on U.S. highways every year.

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If you live in the United States, you probably know the highways around you pretty well, but don’t have a clue once you leave your comfort zone (unless you’re an interstate truck driver or die-hard road tripper). Luckily, there’s actually an order to the highways, and once you know it, it’s much harder to get lost (that’s not a guarantee :).

New Englanders know the I-95, 90, 495. Southern Californians use the 10, the 405, and the 5. But what does it all mean?

Well, the numbers aren’t crazy or random, and, if you zoom out on the map, you can see that there’s actually a great order to them that makes sense.

Quick Guide to the U.S. Interstate System

Quick History of the Eisenhower Interstate System

President Dwight Eisenhower, right, with retired General Lucius Clay in 1955 — President Dwight Eisenhower, right, receives recommendations for a $101 billion federal state highway program from retired General Lucius Clay in 1955. The president and Clay are holding a map of a "strategic network" of interstate highways. Clay was chairman of a presidential advisory committee that drew up a proposed program.

The U.S. Interstate Highway System was developed in the 1950s, spearheaded by President Dwight Eisenhower, to build a strong transport network to connect the country.

Before that, it was pretty haphazard. In fact, the army sent a fleet across the country to test the roads in 1919. It took them 62 days to go from Washington, D.C., to San Francisco.

Guess who was on that trip? Eisenhower.

How the Interstate Highway System Works

Map of the U.S. Interstate Highway system.

While there are lots of weird exceptions, this is how the system works:

All primary main interstate highways — the big boys — are numbered 1-100.

Anything going north-south is odd, starting with 5 on the West Coast all the way to 95 on the East Coast.

Anything going east-west is an even number, starting with 10 at the bottom and 94 at the top.

It’s kind of a grid (more like an abstract artist's version of a grid).

Three-Digit Highway Routes

Highway sign to Long Island — Highway interchange in Queens, New York.

There’s also three-digit highways — like the 495 in Long Island or I-294 in Chicago. Those are local side routes that shoot off of one of the main interstates.

The last two digits call out the main interstate that it connects to. The first digit usually tells whether it connects back to the main highway (even) or whether it shoots off and ends somewhere else (odd).

What About State, County and City Routes?

I-10/I-110 interchange in Downtown Los Angeles — Drone shot of the I-10/I-110 interchange on the edge of Downtown Los Angeles at sunset.

While knowing the interstate number system helps a lot, it doesn’t solve all your problems. States and cities all have their own highways too, and they don’t have to follow these rules. State highways are the ones that usually have an outline of the state (although some states like to get a little fancy).

And the ones that look like this are the older version of the U.S. highway system. They’re not standardized like the interstates.The interstate system always has the blue and red shield.

Enjoy getting lost less.

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