Do interstate numbers have meanings?

[Karen]

I heard a while ago that interstate numbers can tell a little about the road. I remember someone saying that a 3-digit number ending in 5 was a beltway, or a 2-digit number ending in five was a north/south interstate.

That seems to hold true so far in the few (2 to be exact) places I have lived. Is it true for the who US? Are there more of these that I don't know? I am interested in finding out all of them that you all know! Thanks!

[Ed Mathein]

Two-digit even Interstate numbers are always east-west routes [40, 94, 26, etc]. Two-digit odd Interstate numbers are north-south routes [17, 35, 55, 81, etc].

For three-digit Interstate numbers, the first digit indicates what kind of route it is, and the last two digits give the main Interstate that the route is part of. If the first digit of a three-digit Interstate is even, the route is a bypass [or beltline] that connects at both ends to the main Interstate... 894 is a bypass around Milwaukee, part of I-94. If the first number is odd, the route is a spur, where only one end is attached to the main Interstate. 190 is a spur out of Buffalo, NY, that leands to Niagra Falls. It does not connect at both ends to I-90.

Also, the magnitude of the number can give you some idea of where the Interstate is located nationally. I-5 is on the west coast, and the numbers of north-south Interstates gets larger as you go east [5, 17, 25, 57, 81]. I-10 is the southernmost east-west Interstate, and the numbers increase as you move north [10, 20, 44, 70, 94].

I think that's pretty much everything - everything that I've heard, anyway. Hope this helps!

[Mark Sedenquist]

Karen,
It turns out that there are entire groups of folks who study hwy naming systems as a hobby. Some of the best, (that we have found) sites were developed by David "Zzyzx" Steinberg. Check out this one and this one!

Now, you will certainly have more infomation that you may have wanted!

Editor

[imported_Darrell]

Two-digit interstates that end in a 0 or 5 are contiguous across the entire country from east to west or north to south, respectively.

All the other 2-digit interestates are not coast to coast or border to border roads.

[DaveKaye]

I heard a while ago that interstate numbers can tell a little about the road. I remember someone saying that a 3-digit number ending in 5 was a beltway, or a 2-digit number ending in five was a north/south interstate.

That seems to hold true so far in the few (2 to be exact) places I have lived. Is it true for the who US? Are there more of these that I don't know? I am interested in finding out all of them that you all know! Thanks!

Beltways (which are loops) are three digit numbered with first number 2, 4, 6, or 8. The last two numbers are the primary road that the beltway arises from, and should return to. North-south interstates are odd numbered and east west interstates are even numbered. Because of curves, some segments may not run true to the number. For example, I-69 runs east west between Lansing and Port Huron, Michigan. But I-69, overall actually (when completed) will go north south from Port Huron to Texas.

And, on the subject of three digit numbers, I was once told (50 years ago) that the prefix 1 or 3 was for spurs (don't return to any interstate), and 7 and 9 were for connectors - an interstate segment that is spur-like but eventually encounters another (different) interstate such that 2 (or more) different interstates are connected, but not by a complete loop. However, the numbering in Michigan doesn't really follow that. I-696 is not a loop wrt I-96, but is a connector between I-96 and I-75, and I-94. I think it should have been numbered I-796 or I-996. I-196 between Grand Rapids and Benton Harbor should also have a 7 or 9 prefix, if that designation still holds. It is a connector between I-96 and I-94. It could also be I-794 or I-994 for that matter. (It's an old freeway by the way; much of it was there by 1960 already) Finally, I take issue with the designation of I-69 between Lansing and Port Huron. That should be I-196 or I-396. It goes strictly east west, not north south, and should've been a spur off of I-96.

I suspect they eventually dropped the specific numbers for prefixing loops, spurs, and connectors at some point. But I think its a good idea.

[Mark Sedenquist]

Thanks for updating this old thread with new information.

Mark

[Lifemagician]

Maybe this could be linked to the international pages, as I find that very few visitors from overseas have any idea about the numbers - even those who have been on forum.

Lifey