When you’re cruising along one of Chicagoland’s highways, what’s on your mind?

What song to play next? Where the nearest burger place is? When all this construction will be finished?

Do you ever find yourself wondering how all the exits got their numbers?

It’s relatively simple — but not necessarily intuitive.

For interstates in Illinois that run north-south, like I-55, the numbering system starts at the southernmost point the highway enters Illinois. For interstates running east-west, like I-80, the numbers start at the westernmost point.

A lot of times, the very first turnoff is Exit 1 — but the next one isn’t necessarily going to be Exit 2.

That’s because the numbers correspond not to how many exits there are, but to how many miles away from the border they are. So by following the exit sign numbers, you can actually track how far you are from the state’s boundary line.

Sometimes things get a little tricky. Where two interstates merge, the numbering system for one supersedes the other. For example, if you’re heading downtown along I-90 where it meets I-94, the exit numbers jump from 84 down to 44.

Which brings us to one more wrinkle in the system. Around the city, I-90 — that’s the Skyway, Dan Ryan, Kennedy and Jane Addams — and I-94 — or the Kingery, Bishop Ford, Dan Ryan, Kennedy, Edens and the Tri-State Tollway — mostly run north and south. But the signs all say east and west. What gives?

Well, don’t forget about the inter part of interstates. Our highways are part of a nationwide system, and I-90 and I-94 broadly run east-west across the country. They’ve just got to curve around pesky ol’ Lake Michigan to carry on their merry way, so they move north-south through the Windy City.

So the next time you find yourself stuck in bumper-to-bumper traffic, you may not know how long you’ll be sitting there, but at least you’ll know how far your bumper is from the border.

Contact Nick Blumberg: [email protected] | (773) 509-5434 | @ndblumberg