
The Math Inside the US Highway System - skolos
https://betterexplained.com/articles/highway-math/
======
mikestew
"However, the numbers follow a grid, with rules nobody told me."

I learned it from reading a state map about forty years ago. The legend
succinctly explained the system. Without digging through a pile of paper maps,
that might still be the case. Regardless, these rules are probably clearly
documented from a simple web search, despite that no one has felt the need to
"tell" the author what the rules are.

EDIT: "interstate numbering system" on DDG brings up the Wikipedia page as the
first link, and that link explains it. There's no mystery here, though my
query string does assume that one assumes a "system" to be explained.

And while I'm editing, state routes and U. S. Routes kind of follow the same
pattern, especially the N/S and E/W designations. Would have to go search to
verify other commonalities.

~~~
minikites
I have some sympathy for people who were never told this or who are too young
to have had a reason to encounter it. I work in a university and the number of
times I've been asked, "What floor is room 302 on?" by new students is greater
than zero.

~~~
jessaustin
I think I've been in buildings where two-digit rooms were on the ground floor,
and so 302 would have been on the "fourth".

Although one would expect university students not to need help with this...

~~~
projektir
I live in an apartment where the numbering system varies depending on the
building, one of them has 11-- for the first floor, 12-- for the second, 13--
for the 3rd, and I think 10-- is "Terrace" level.

> Although one would expect university students not to need help with this...

Rather curious why you felt the need to make a comment of this nature.

~~~
jessaustin
To satisfy your curiosity, I meant only that one would assume admission to
university would require a modicum of intelligence or perhaps even awareness,
which intelligence and awareness would amply suffice for the task of reading
basic hallway signs.

~~~
projektir
I always found the it interesting that when somebody tries to find something
to feel better than someone else about, and they pick the most basic, mundane
thing. Tying shoes, adding small numbers, reading hallway signs.

~~~
jessaustin
The scales have fallen from my eyes!

------
eggy
I was aware of most of this, but it segues nicely with my latest interest in
Adam Adamatzky's research using the slime mold, Physarum polycephalum, in a
form of bio-computing [1].

The demos where oatmeal flakes are placed on a highway map of major cities in
various countries, and the Physarum optimize the paths in ways that closely
resemble some of the existing major roads is compelling. I am holding out on
how correlated it actually is given highways have developed not only from old
horse paths and such, but from civic planning.

My daughter is a Chemistry/Bio major, and I have told her about this, and
asked her advice on how I might go about doing this at home. It seems easy
enough under controlled circumstances, and it it just seems fun checking in
hours later for a 'result'! So much so, I ordered his book "Physarum Machines:
Computers from Slime Mould".

[1]
[http://uncomp.uwe.ac.uk/adamatzky/research.html](http://uncomp.uwe.ac.uk/adamatzky/research.html)

~~~
adrianN
I did some work on Physarum. It is indeed fairly easy to do simple experiments
with the slime mold. You just need some petri dishes and a dark, moist
environment. Just don't expect nice results every time, sometimes the slime
mold just growths in a completely unexpected way.

------
Sanddancer
There's another rule this one doesn't touch. Route numbers that are divisible
by 5 are considered major routes, and with a few exceptions go from one
geographical extreme to the other -- Interstate 5 goes from Canada to Mexico,
Interstate 80 goes from San Francisco to New York.

~~~
jedberg
> Route numbers that are divisible by 5 are considered major routes, and with
> a few exceptions go from one geographical extreme to the other

They tend to be major, but only 11 of 19 (11 of 17 that exist) go from one geo
extreme to the other. Sorry highway numbers are a hobby of mine (yes I know
strange hobby).

5 ^: Mexico to Canada

10 ^: Pacific to Atlantic

15 ^: Mexico to Canada

20: Mid-Texas to Mid-South Carolina

25: Southern Texas to Wyoming

30: Texas to Arkansas

35 ^: Mexico to almost Canada

40 ^: Eastern California, to Eastern North Carolina

45: Entirely in Texas

50: N/A

55 ^: Gulf of Mexico to Great Lakes

60: N/A

65 ^: Gulf of Mexico to Great Lakes

70: Utah to Maryland

75 ^: Southern Florida to northern Michigan

80 ^: San Francisco to New York

85: Mid-Alabama to Mid-Virginia

90 ^: Seattle to Boston (longest in the country)

95 ^: Maine to Florida

~~~
chiph
I-40 is an interesting case. It hits the ocean in Wilmington NC, but doesn't
make it to the coast in California, ending in Barstow. The completionist in me
says that it ought to continue through Palmdale to Ventura.

Until a few years ago, it wasn't contiguous. There was a gap in Memphis where
the locals objected to its completion because the traffic noise would have
disturbed the animals at the zoo. So the northern part of I-240 was signed to
include I-40, finally linking the two halves together.

~~~
amyjess
> There was a gap in Memphis where the locals objected to its completion
> because the traffic noise would have disturbed the animals at the zoo.

A bigger reason is that the road would have run straight through Overton Park
and required that the entire park be destroyed. People like Overton Park, and
the protests over that got the project cancelled. In fact, they actually built
the road east of Overton Park, but they stopped there when the protests
started. What was intended to be I-40 is now called Sam Cooper Blvd. If you
look at Google Maps, you'll see that the gap between the end of I-40 on the
west side and the beginning of Sam Cooper basically just consists of Overton
Park and a few blocks in each direction.

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GCA10
I'll agree with @Mikestew that this is pretty familiar territory. But it did
get me looking to see how well the original Interstate planners did, in terms
of leaving unallocated numbers available for future expansion.

Surprise, surprise: regional economies expand at different rates, which
forecasters don't always get right. It's not as messy as Social Security
numbers or area codes, but there are definite pockets of congestion.

For example, the East-West corridors between I-80 and I-90 have been graced
with more highways than expected. We've had to come up with two versions
apiece of I-84, I-86 and I-88 in different parts of the country, along with
one I-82. By contrast, there's been no East-West build-out whatsoever between
I-30 and I-40.

More details are here:
[https://en.wikipedia.org/wiki/List_of_Interstate_Highways](https://en.wikipedia.org/wiki/List_of_Interstate_Highways)

~~~
labster
As to how we're planning to use up the numbers for future expansion, there's
this Wikipedia page:
[https://en.wikipedia.org/wiki/Future_Interstate_Highways](https://en.wikipedia.org/wiki/Future_Interstate_Highways)

Of course if you want to take the I-7 to Fresno, the future is now because
it's called 99 but not yet built to interstate standards.

~~~
amyjess
Which is funny because once that's done, that road will have been a US Route,
a State Route, and an Interstate _in that order_.

SR 99 was originally part of US Route 99, but when I-5 was built AASHTO
decided to decommission Route 99 since the whole thing paralleled I-5 so
closely that it was considered redundant. The states, however, decided to
continue to maintain (most of) the road, and it was assigned a number of state
highway designations, including SR 99 in the Central Valley (the states,
particularly Washington, did give up on a few portions of it, which are now
maintained as local roads in various cities).

And now they want to make it an Interstate.

(Also, speaking of future Interstates, I'm kinda shocked that I-66 was
cancelled. They upgraded a bunch of roads to Interstate standards, and they
even signed the Cumberland Pkwy. in Kentucky as "Future I-66 Corridor" (you
can even see it on Google Street View), only to give up on it.)

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chucksmash
It's always interesting to see what gets posted and upvoted. Growing up before
people generally had access to the internet, (when you had to use a physical
map to find places), I'd heard these particular tidbits repeated ad nauseum
every time our family took a road trip.

I wonder:

a) is this new to anybody?

b) if so, what other tidbits of "everybody knows that" are fading into
obscurity because of technology? (Surely you all remember how to reshoe a
horse in a pinch at least?)

~~~
jenaimarre
a) yes, it is new to me because I was not raised in your country, not because
the web came after I was born.

------
DrScump
The fun part is when you're traveling westbound on I-580 in Berkeley, CA...
_and_ you're simultaneously going _eastbound_ on I-80... on the _same stretch
of road._

~~~
catbird
Or the place near Trenton, NJ where I-295 North becomes I-95 South, and vice
versa.

------
ibmthrowaway218
The UK's roads have a numbering scheme:
[https://en.wikipedia.org/wiki/Great_Britain_road_numbering_s...](https://en.wikipedia.org/wiki/Great_Britain_road_numbering_scheme)

------
GlennS
They encoded real information into their primary key. They're going to run
into trouble when they need to change one of these roads later.

(Like maybe rotate one of these North-South roads to run East-West.)

~~~
jrapdx3
Road direction can be ambiguous. Fairly obvious with a loop like I-270 around
Columbus, Ohio. I've thought they should have designated the direction as
clockwise and counterclockwise, going around the loop for a significant
portion gets confusing re: "North" which may be actually east or west.

Also I-84 in western Idaho is more north-south even though it's even-numbered
and nominally and east-west highway. Probably not too confusing because roads
are much more sparse in that part of the country, people navigate on the basis
of destinations, the exact direction is kind of hypothetical.

No surprises in it though, real-world roads won't follow the abstract grid
very closely, there are going to be quite a few exceptions.

~~~
WorldMaker
Louisville, KY has a definite hub and spoke layout with both I-264 (East/West)
and I-265 (North/South) as increasingly outward rings (and several state
routes and major parkways are additional "rings" in between). Many natives
don't know the cardinal directions of the interstates or state routes and
there's a lot of landmark-based navigation (towards the Fairgrounds/Airport,
away from Downtown, et al). I like using clockwise/counter-clockwise
directions myself and after an initial bit of confusion it's fun when fellow
natives blink a bit and recognize it's not a bad system. (Thus far haven't
made any converts to it, though.)

Further aside, given the the similarity in numbers of I-264 and I-265 they are
more commonly referred to by their expressway names, the Watterson and the
(Gene) Snyder, respectively. Louisville is one of the few cities I know of
where you'll meet natives that don't actually know the interstate numbers they
drive on sometimes every day, you have to mention it by name to get the right
directions.

------
dajohnson89
Why do the Boston and Washington Beltways have the same number (495)?

[https://en.wikipedia.org/wiki/Interstate_495_(Massachusetts)](https://en.wikipedia.org/wiki/Interstate_495_\(Massachusetts\))

[https://en.wikipedia.org/wiki/Interstate_495_(Capital_Beltwa...](https://en.wikipedia.org/wiki/Interstate_495_\(Capital_Beltway\))

~~~
douche
I-495 used to be the name of the Maine Turnpike, as well, before 2004. Now it
that route is included in I-95, while the previous I-95 is now labeled I-295,
while a yet third section of highway connecting the two above Portland is now
technically 495.

~~~
24gttghh
But I-495 is not signed as such; it is simply called the 'Falmouth Spur' ;)

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bbanyc
Some states (e.g., Indiana) have grids similar to the Interstate and US
routes. Others have different systems: in Florida the first digit tells you
roughly where in the state it is, from 1xx routes in the Panhandle to 9xx
routes around Miami. And still others just assigned numbers in order of
construction, so on a map it makes no sense at all.

------
maus42
I wouldn't call it very math-y, I'd call it a systematic numbering scheme.

------
pklausler
I-82 in Washington State is a glaring exception to the numbering convention,
being located north of I-84. Are there other exceptions?

~~~
Sanddancer
Another glaring exception is Interstate 238 in California. It's an important,
if short, route in the Bay Area, but at the time it was redesignated an
Interstate, every x80 number had been taken, either by other local freeways,
or by unrelated highways. So, when it was updated to interstate standards,
they just decided to let it keep its old number, even though interstate 38
doesn't even exist.

Since then, Highway 480 (it was never formally adopted as an Interstate) has
been removed, but renumbering would be more trouble than it's worth, so it's
remained I-238.

~~~
GFK_of_xmaspast
There's also I-595 between DC and Annapolis, which is signed as US-50 instead
because there are too many x95s in the area.

~~~
Sanddancer
There's a simpler explanation. It's a pre-existing route that was adopted by
the federal highway system. US 50 is well known enough that resigning it would
be more trouble than it was worth.

~~~
GFK_of_xmaspast
Interstates don't usually spring out of thin air, they're often existing US
Highways or state roads that get relabeled or upgraded to interstate standards
and relabeled.

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whybroke
Except where the numbering is magical such as the renaming of hwy666 when some
groups suddenly decided the number was satanic[1]

[1]
[https://en.wikipedia.org/wiki/U.S._Route_491](https://en.wikipedia.org/wiki/U.S._Route_491)

~~~
amyjess
Route 666 was renamed because people kept stealing the signs for novelty
value, and it was costing the state too much money to keep replacing the signs
again and again and again. IIRC, they even rolled out signs that were
supposedly "theftproof"; they got stolen _anyway_ , and it cost the state a
lot of money to develop them.

~~~
whybroke
And vandalism btw. Particularly the preparation needed to remove a 'theft
proof' sign is not done by an idle souvenir collector on a road with essential
no tourists that no one form outside the area has ever heard of. Those things
are just as likely done by the few who have a moral objection to the number.
Ironically, the road could have been turned into a minor tourist attraction
for Gallup if not Shiprock. Likewise, copies of the sign could have been
pedaled in Gallup to feed any souvenir collectors as is done with rte 66
signs.

But most interesting to me is the way the attitudes of evangelicals concerning
the spiritual status of the valley (albeit newly acquired and negative)
spilled over into the Navajo beliefs as the area become known on the
reservation to have a high number of Skinwalkers.

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mindslight
Metro Boston is a fantastic region with which to illustrate the flawless order
of the highway system's numbering!

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lazyant
more an encoding than "math"

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dsfyu404ed
The US interstate numbering system is fairly common knowledge. Does it by
itself really deserve a post here?

~~~
ryandrake
Perhaps this is interesting to folks outside of the US. It's common knowledge
to US people only.

