What Do the Numbers on Interstate Signs Mean?

American interstates aren't given those numbers randomly. Learn how information is coded into every number -- and how this high-speed road system got its start -- in today's episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio Pay
brain Stuff Lauren Boba blam here. If you've ever driven
west on Interstate A D through Nebraska, you've seen one
of the flattest and emptiest roads in America, and you
probably had some time to ponder life's most important questions.
For example, why is the highway called I A D anyway?

(00:26):
And why do different interstate names contain one or two
or three digits. Well, buckle up, because you're about to
learn about the codes behind the US interstate naming system.
But for starters, let's talk about how the interstate highway
system began. The formal name for these roadways is the
Dwight D. Eisenhower National System of Interstate and Defense Highways,

(00:48):
and the project was approved by Congress through the Federal
Aid Highway Act of ninety six that authorized the construction
of about forty one thousand miles or sixty six thousand
kilometers of highways stretching across the country from east to
west and north to south. One of the main purposes
of the interstate system was national defense. President Eisenhower had

(01:09):
served as the Supreme Commander of Allied Forces during World
War Two. In Europe and had witnessed the importance of
Germany's Autobahn network, which allowed for speedy transportation throughout the country.
Another slightly more ominous selling point the quick evacuation of
cities in the case of potential nuclear attacks. And of course,
the system was touted as a way for all Americans

(01:31):
to travel with fewer traffic jams and more efficient routes
with no at grade crossings, that is, intersections. The system
used overpasses and underpasses to allow for seamless, stop free,
high speed travel. There are now forty six thousand, eight
hundred and seventy six miles or seventy thousand, four hundred
and thirty nine kilometers of roadway within the interstate designation.

(01:56):
Construction began in nineteen fifty six in Missouri on what's
out named I seventy, but the system as a whole
wasn't completed until when crews finished a devilishly difficult stretch
in Glenwood Canyon, Colorado. There you'll find forty bridges and
viaducts on a single twelve mile or nineteen kilometer stretch.

(02:17):
The Interstate Highway System transformed American life. The cargo trucking
industry arose, gas stations and truck stops, blossomed, suburbs sprung
up around cities, and the concept of the road trip
entered the national lexicon. And as those road trippers zipped along,
some of them surely wondered why interstates have various numbering systems.

(02:38):
But why is one named I five while another is
I four eighty. Let's take a closer look at the
digits on those shield shaped white, red and blue signs.
The number of digits in the name tells you whether
an interstate connects more than one city, or if it
serves a single metropolitan area. Interstates with one or two

(02:59):
digits all interconnect several regions. Those with three digits, like
I to eighty five, on the other hand, are meant
to serve a single city and are called auxiliary interstate highways.
They connect to longer two digit highways. The last two
digits match the parent highway. For example, I four e

(03:19):
d in Omaha is a five mile stretch that connects
I e ight in Nebraska with I twenty nine in Iowa. Likewise,
the aforementioned I T eighty five circles Atlanta and connects
to I eight five going north and south. Continuing to
use that as an example, you can tell that I
A D five runs north to south because odd numbered

(03:40):
interstates move traffic north to south, while those with even
numbers stretch east to west. The numbering for east west
highways starts in the south and moves north, so I
ten anchors the bottom border Santa Monica, California, to Jacksonville, Florida,
and I ninety is near the country's northern border, a
Seattle to Boston. Meanwhile, for north to south running interstates,

(04:03):
numbering begins in the west, starting with I five parallel
to the west coast. The easternmost interstate is I N five,
running on the east coast from Maine to Miami. There
are some rare exceptions to this pattern in cases where
stretches of road were added after the first framework was
already in place. For example, I N nine didn't receive

(04:25):
its interstate designation until and so it lies to the
west of There are only two places across the country
where interstates split as they moved through major cities, both
on I thirty five, one around the Minneapolis St. Paul
metro area and the other to go through Dallas and
Fort Worth. Meanwhile, the first digit of a three digit

(04:48):
interstate tips you off to its purpose. Connectors or spurs
that only intersect one time mostly receive odd numbered first digits.
Even numbered first digits, how ever, tend to be bypasses
and loops that intersect with their parent interstate in two locations.
The names of major interstates are never repeated. They're always unique. However,

(05:11):
some lesser stretches do use the same two digit numbers.
For example, both New York and North Carolina have an
I eighty seven. There are also I seventy six is
in both Colorado and Pennsylvania. Three digit interstates, however, can
be used as often as necessary throughout the country, although
they can't be repeated in a single state. But this

(05:33):
numerical interstate system can get even messier. For instance, in
San Francisco, there's I to thirty eight, even though there's
no I thirty eight for it to connect to. Instead,
it's an artery between I e eighty and I five eight.
But because California already had so many roads, the more
appropriate digits were already in use with other roadways, and

(05:57):
there are other exceptions to Alaska, y E, and Puerto
Rico all have interstates Even though these roads don't connect
to other states, they have special lettered designations. Alaska has
A one through A four Hawaii and H one through
H three in Puerto Rico PR one and PR two,

(06:17):
and state highway naming convention differs a bit from the interstates.
These roadways, which are denoted by black and white signs
with badge symbols, use the same system of odd numbers
for north south routes and even numbers for east west routes. However,
the number system is flip flopped. The numbers on east
west roads gets smaller as you go from south to north,

(06:38):
and the numbers on north south highways increase as you
go from east to west across the state. Meanwhile, I
ninety is the longest U S inter state, stretching all
the way from Boston to Seattle and covering nearly three thousand,
one hundred miles or five thousand kilometers, traverses thirteen states,
and I, which connects Miami to Maine, cuts through the

(07:01):
most states of all the Inner States fifteen, and it
also goes through Washington, d C. Today's episode was written
by Natan Chandler and produced by Tyler clang Or. More
on this and lots of other numerous topics. Visit how
stuff works dot com. Brain Stuff is production of iHeart
Radio or more podcasts to my heart radio, visit the

(07:23):
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