Those small green signs counting up the side of the highway are mile markers, and they're more useful than most drivers realize. Here's how to read them, what the numbers mean, and how to know your exact one even when you can't see a sign.

What the number actually means

A mile marker tells you how far along a highway you are. On most US interstates, the count starts near mile 0 where the route enters the state, at the south or west border, and rises as you head north or east. So mile marker 120 means you're roughly 120 miles from that starting point, and the next state line resets the count.

Odd or even tells you the direction

The route number itself is a clue. Odd-numbered interstates run north to south, like I-5 up the West Coast and I-95 up the East Coast. Even-numbered interstates run east to west, like I-10 across the south and I-90 across the north. The numbers grow as you go: low odd numbers sit in the west, low even numbers sit in the south.

That's also why mile markers count the way they do. On a north-south (odd) route, mile 0 is at the southern line and the numbers climb as you head north. On an east-west (even) route, mile 0 is at the western line and climbs as you head east. So the route number and the mile marker together tell you both where you are and which way you're pointed.

How to read the signs

Most states post a sign at every whole mile. Some post them more often, every two-tenths of a mile, on busier or higher-risk stretches. The signs are small on purpose, so scan the right shoulder as you drive and you'll see the number tick up (or down, if you're heading south or west).

They also tell you how far to your exit

On most interstates, exit numbers are based on mileposts: Exit 57 sits at about mile 57. So if you just passed mile marker 47 and you want Exit 57, you have roughly ten miles to go. That's why the milepost is a better "how far" answer than "near exit 12," which is vague across a six-mile gap.

Reference: Mileage-based exit numbering

What to do when you can't see a sign

Signs can be a full mile apart, and they're easy to miss at night, in weather, or when you're focused on traffic. Your phone's map app won't help here: Google Maps and Apple Maps don't display mile markers, and there's no setting to turn them on.

That's the gap MileCheck fills. It reads your GPS position and shows your nearest mile marker in real time, interpolated to a tenth of a mile, so you see 34.4 rather than just the last sign you passed. The marker data is built into the app, so it works even with no cell signal.

Why it's worth knowing

The mile marker is the location format the people who run the roads actually use. It's what 911 and tow dispatch ask for when you break down, what a friend needs when you say "meet me at this spot," and what turns "somewhere on I-90" into an exact place.

On rural roads it's often the only way to point at a place with no street address. "The first driveway on the right after mile marker 55." "The trailhead at mile marker 12." "Cows are out on the fence line near mile marker 88." The mile marker is the address when there isn't one.

It works the other way too. You're driving I-90 and a traffic report says there's a crash "between Seattle and Cle Elum." That's a good eighty miles of highway. Where exactly? A mile marker would have told you in one number whether it's ahead of you or already behind. Radio reports, DOT signs, and dispatchers all lean on mile markers for the same reason: it's the only way to describe a spot on the road out loud, when there's no map to point at and no pin to send. Knowing yours takes the guesswork out of the moment you need it most.

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Essentials: What to tell 911 on the highway  ·  Browse the live mile marker map  ·  All 50 states