Common Tread

Why things are the way they are: The highway numbering system

Aug 13, 2015

I love two-laners.

It's the only way to fly on a motorcycle, if you ask me. I don't care where I'm going, but I like to be on a thin, blacktop road, ideally with no one around. However, the double-yellows usually mean there's oncoming traffic, and oncoming traffic means that I can't really put my hammer down. Really fast roads have medians or dividers and no intersections because the key to making time is not stopping.

Sometimes, you have to be somewhere in a hurry. Long weekends leave you 500 miles from home on a Sunday. Sometimes the sun dips faster than you thought, and you're still 100 miles from where you planned to be, and you still have to make camp. Sometimes you just need to get onto a freeway and burn to get the odometer rolling. I've done this before with no GPS or map to aid me, because I know what the U.S. Interstate Highway System numbers mean.

Interstate highways here in the United States have an orderly numbering system. I know smartphone-less travel might seem antiquated to some of you, but some folks ain't got 'em, and if your GPS dies or your directions blow off the tank or your cell phone service evaporates, it doesn't hurt to have a contingency plan in the form of a little knowledge.

Interstate highway sign
It's amazing how much information is on such a small sign. You are riding east on I-80, one of the major, transcontinental interstates, because it ends in zero, and the state line is 132 miles in your rearview mirror. Photo by Lemmy.
Interstate highways, the ones with the numbers shown on a blue shield with a red stripe atop it, are assigned a one- or two-digit number. East-west roads have even numbers and north-south highways have odd numbers. Numbers start small in the west and south and rise as you move east and north. The major, transcontinental ones end in zero, like I-10 in the south or I-90 in the north. (Most of the country calls the interstates I-40 or I-95, for example, but if you're a cool Californian like Sean, you have to say "the five" or "the four oh five." Did I do it right, Sean?)

Branches off these main routes have three-digit numbers with the last two digits being the parent road. An even first digit usually designates a road that loops around a major metropolitan area, such as I-495 around Washington, D.C. An odd first digit is a spur road, branching off from the major interstate to get traffic to a separate location, such as I-110 linking Los Angeles to I-10. While the numbers on the main roads are not repeated, branch roads are. When I was in college tooling around on my old Honda, I would jump onto I-270 to avoid St. Louis traffic, and Lance does the same on a different I-270 to get around Columbus, Ohio, near his home. If you say "395" to me, I think of the road I've traveled in Connecticut, but my little brother rides his Kawasaki Z1000 on I-395 through our nation's capital.

Here you can see I-495, a loop that bypasses Washington, D.C. Also note I-395. The odd first digit indicates this is a spur, in this case connecting points in Virginia to I-95 in Washington. Google map.

There are some exceptions to these rules, like I-99 in Pennsylvania, due to political moves, but they are relatively few. Also, despite what you've heard, the Interstate Highway System was not designed with planes making emergency landings in mind.

U.S. 40
The U.S. highways predated the interstate system and used a similar numbering scheme, but reversed. Numbers are shown on a white shield. The major, transcontinental routes ended in zero. But unlike the more modern, limited-access interstates, U.S. highways passed through every town, making for slow going. U.S. 40 was also known as the National Road. Photo by Lance Oliver.
The Interstate Highway System followed the older U.S. highways, which were numbered in reverse: numbers rise as you move from east to west and north to south. So U.S. 1 runs the length of the east coast, as does I-95. That's also the reason there's no I-50: a desire to avoid confusion, since U.S. 50 and I-50 would be in the same states.

Now here's maybe the most useful feature for those of us riding with small tanks. Most states are switching over to a system where the exit numbers match up with the mile markers. Mile markers start at zero when you enter a state and rise as you go east or north. Thus, if you're at exit 35, and the next exit on the map is exit 61 and you hear only a mouthful of fuel sloshing in your tank, you probably ought to pull over for fuel. Most riders know this and are probably saying "duh!" right now, but if there's even one rider who learned something with that gem, it was worth having in here.

And that's the whole mystery unraveled. So when you see those numbers on the signs, now you know why things are the way they are.