Interstate numbering explained: (or why there is no I-50 or I-60)

Open Road

"Eliza, why is there no I-50/60, why?" someone once asked (though I suspect he knew the answer already).

In 7th grade I wrote a report about the design, history, and numbering of the Interstate system. In the spirit of recycling knowledge, here are the highlights as I recall them:

Major routes end in either a 5 or a 0
North/South routes are odd numbers
East/West routes are even numbers
Odd numbers begin on the west coast (I-5) and increase as you travel eastward (I-95 along the east coast)
Even numbers begin in the south (I-10) and increase as you travel north (I-90 which parallels the Canadian border)
A three digit route is a belt-route (1xx, 2xx, 3xx, 4xx, etc.)
If the first digit of a three digit route is even, the belt-route typically forms a loop
If the first digit of a three digit route is odd, the belt-route is a typically a spur
I-50 and I-60 do not exist because they would run through areas served by US 50 & US 60

The Federal Highway Administration's Route Log and Finder List has more information.

An open question (as far as I know), in the spirit of The Traveling Salesman Problem, is: what route you would need to take on a road trip to drive at least one mile on each numbered Interstate (either with or without including beltroutes) in the 48 contiguous states to minimize distance traveled and/or time.

In Utah the quickest way I can think of to go at least a mile on each Interstate would be to begin at Cove Fort near the I-70/I-15 junction. Travel on I-70 and go north on I-15 at the junction. Continue until you get to the southern I-215 interchange in Salt Lake County and take it to I-80. Go up into the canyons until you reach the I-84 junction near Echo.

(Interstate 50 is also the name of an interesting artistic photo collection.)

—Michael A. Cleverly
Tuesday, July 19, 2005 at 00:29

Comments:

John Cowan wrote (at Tue, 19 Jul 2005, 11:36):

All those rules and a lot more (like, why is there an I-238 when there is no I-38, and what on Earth is it doing in California?) explained at http://www.ihoz.com/interstates.html .

The U.S. highway numbers are less systematic, but they do have the rules north-south odd and east-west even, though they are numbered from east to west and north to south respectively (U.S. 1 and U.S. 2 intersect in Bangor, Maine).

Louis Zirkel wrote (at Tue, 19 Jul 2005, 13:12):

Paula and I recently watched a History Channel program about this very subject and I remember part of the rational behind the number scheme, and more importantly the direction (west to east ascending, north to south descending) was to avoid as many clashes (i.e. the US50/US60 vs I-50/I60) as possible.

Nadine Lagarde wrote (at Fri, 22 May 2015, 17:59):

How many cities in the U.S. are there that have two interstates that cross each other?

How many cities in the U.S are there that have three interstates that cross each other?

Thank you

Derek Mohr wrote (at Wed, 11 May 2016, 01:12):

None all interstates turn into other interstates they don't intersect. They have bypasses that "bypass" large cities. 275 bypasses Tampa for example. So I don't believe that there are any cities where interstates intersect. If I'm wrong please let me know. I'd be interested to find out if my thesis is correct. Thank you.

Isaac Levy wrote (at Fri, 17 Jun 2016, 20:34):

Seriously Derek, there are plenty of Interstates that intersect inside of major cities in the US. For example, I-5 and I-10 intersect near Downtown Los Angeles. I-30 and I-35W interects in Downtown Fort Worth. The list goes on and on, but plenty of mainline Interstates intersect in major cities.

Isaac Levy wrote (at Fri, 17 Jun 2016, 20:43):

Derek, there are plenty of Interstates that intersect inside of major cities in the US. For example, I-5 and I-10 intersect near Downtown Los Angeles. I-30 and I-35W interects in Downtown Fort Worth. The list goes on and on, but plenty of mainline Interstates intersect in major cities. There are bypasses, such as I-610 in Houston and I-495 around the Washington DC Metropolitan area, among many others, but those are used to reach and connect the outskirts of major cities and sometimes to divert 18 Wheelers and other commercial vehicles away from the downtown area of cities to lessen traffic. Some cities and metropolitan areas are large enough to require multiple loops & secondary interstates to handle traffic, but that's another story...disregard my first comment, sorry for coming off harsh

Ash wrote (at Sun, 27 Nov 2016, 20:47):

Which interstate has the best lot lizards at the rest stops?

PFunk wrote (at Mon, 25 Jun 2018, 15:06):

I have a notebook somewhere where I answered that exact question. I scrolled google maps and found every interstate intersection in cities. I think it was something like 92, but at the time I didn't count auxiliary highways. I should recount that...

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