The exit number is the mile marker

There is a piece of information printed at the side of every American interstate that people drive past for years without reading, and it does the one thing a road sign is rarely asked to do: it tells you exactly where you are.

On most United States interstates the exit number is the mile marker, so the difference between two exit numbers is the distance in miles.

Exit 243 to exit 263 is about twenty miles — the numbers round to the nearest marker, so it is close rather than exact.

That is immediately useful in a small way — you can answer “how far to the services” without asking a machine — and it is useful in a much larger way, which is above this page and is the reason it is worth knowing.

The small green mile markers themselves run the whole length of the road, counting up from the state line as you travel north or east. They also tell you something quietly: when the number resets to a low figure, you have crossed a state line.

The numbers themselves

The rest of it is the pattern people half-know, and it is worth having straight because the half-known version misleads.

Odd-numbered interstates run north to south. Even-numbered ones run east to west. The numbers count up from the bottom left of the country — the low odd numbers on the west coast, low even numbers in the south — so I-5 is on the Pacific and I-95 is on the Atlantic, I-10 runs along the southern edge and I-90 across the top.

Three-digit interstates are the ones people get wrong, and the correction is specific: it is the first digit that tells you, rather than whether the whole number is odd or even. An even first digit means a loop — it leaves the parent route and rejoins it. An odd first digit means a spur, which goes somewhere and stops. The last two digits are the parent road, so I-495 is attached to I-95 and I-270 to I-70.

US Highways — the older numbered routes, not the interstates — use the same odd and even logic and count from the opposite corner, starting in the northeast. Which is why a road numbered 1 and a road numbered I-5 are at opposite ends of the country.

Where it stops being true

This is where the confident version of this advice does damage, and the people who drive for a living were firm about it.

The rule is for the Interstate System. State routes do not follow it, and there are a great many state routes with highway numbers on them. Some places are notorious: roads that run east-west with odd numbers, three-digit routes that behave like neither a loop nor a spur, one number appearing in three different states doing three different things. Parts of the Northeast do not use numbers for roads in conversation at all.

So it is a reasonable expectation rather than a fact you can act on. If the road is an interstate and the number is behaving oddly, the number is probably the thing that is wrong.

And somebody made the sharpest objection to the whole subject, which is worth carrying: knowing which way a road runs does not help if you do not know where you are. That is entirely right, and it is why the mile markers are the part of this actually worth learning. The direction is trivia. The number at the side of the road is a location.

Which is also why this used to matter more. A generation ago, this was how you worked out where you were with a paper map on the passenger seat. Now a phone does it — right up until the phone has no signal, which on the long empty stretches is exactly where it happens.