How are 3 digit interstate #'s determined?

[dvferyance]

I always figured they just started at the lowest number (1xx or 2xx) then went up as more were added.  Occasionally a number may be skipped - a planned route never occurred, or an adjoining state already had the low route number so they used the next one to avoid confusion.

Tell that to I-820

Wasn't that State Route 820, then they gave it an upgrade to I-820?  I believe that's the backstory there as to how it was given an unusually high number.

There sure is because radio station WBAP is on AM 820.

[TheStranger]

1.  So basically when choosing which even or odd 1st digit, there is no rule, things vary by state?

From what I've seen in the thread, this seems to be the case.  Some use middle or higher numbers to leave room for lower numbers in the future sequence (i.e. California starting out with 405 and 605 and 805, before creating 205), others do it purely in order (the odd 3dis in South Carolina), and some definitely are more "this number is available, why not use it" (I-820 in Texas stands out, as does I-985 in Georgia).

2.  Why is the DC beltway 495 and the Baltimore 695?

Could this be "Maryland numbering things in geographic order" - 295, 495 both in the DC area, 695 then a bit up north?

3.  If a 3 digit interstate ends at another interstate it must be even, correct, EVEN if that interstate is not an offshoot of 2 digit (Fictionally 387 starts at 87 and ends at 684)?

Not necessarily (see I-355, the Kansas I-135, the under-construction I-369, I-505).  Note that 135 and 505 replaced suffixed Interstate routes (35W and 5W).

[Rothman]

1.  So basically when choosing which even or odd 1st digit, there is no rule, things vary by state?
2.  Why is the DC beltway 495 and the Baltimore 695?
3.  If a 3 digit interstate ends at another interstate it must be even, correct, EVEN if that interstate is not an offshoot of 2 digit (Fictionally 387 starts at 87 and ends at 684)?

1) Yes.
2) Sometimes, there is little reason behind the numbering.
3) No.

[vdeane]

On thing that's worth noting is that 3di numbers don't duplicate within a state, and I-495 exists in both MD and VA (and a tiny piece of DC).  Thus, I-295 for Richmond, I-495 for DC, and I-695 for Baltimore was somewhat dictated by the fact that I-495 exists.

[TheStranger]

On thing that's worth noting is that 3di numbers don't duplicate within a state, and I-495 exists in both MD and VA (and a tiny piece of DC).  Thus, I-295 for Richmond, I-495 for DC, and I-695 for Baltimore was somewhat dictated by the fact that I-495 exists.

DC/MD do have the I-295 along the Anacostia River (which connects to DC 295/MD 295/Baltimore-Washington Parkway as one long corridor).

[vdeane]

On thing that's worth noting is that 3di numbers don't duplicate within a state, and I-495 exists in both MD and VA (and a tiny piece of DC).  Thus, I-295 for Richmond, I-495 for DC, and I-695 for Baltimore was somewhat dictated by the fact that I-495 exists.

DC/MD do have the I-295 along the Anacostia River (which connects to DC 295/MD 295/Baltimore-Washington Parkway as one long corridor).

Which notably does not enter VA.  Similarly, Baltimore has a short I-395 while VA and DC have a longer one.  And I-195 will exist separately in all three jurisdictions (while DC will lose its I-695) if/when the I-395/I-695 renumbering happens.

[freebrickproductions]

1.  So basically when choosing which even or odd 1st digit, there is no rule, things vary by state?

Even vs. odd for the first digit is typically determined by the use the route will serve, IIRC. Odd numbers indicate it's a spur (for example: I-565 in Alabama) while even numbers indicate it's a bypass (for example: I-840 in Tennessee) or a beltway (for example: I-285 in Georgia).

[bluecountry]

So if a road is going from say I-66 north to I-70, could it be either even or odd?  Would it depend if the road ends at the interstates (starts at 66 ends at 70) or if it continues beyond one or both?

[Rothman]

So if a road is going from say I-66 north to I-70, could it be either even or odd?  Would it depend if the road ends at the interstates (starts at 66 ends at 70) or if it continues beyond one or both?

It could be any number the State puts forward at AASHTO. 

[1995hoo]

So if a road is going from say I-66 north to I-70, could it be either even or odd?  Would it depend if the road ends at the interstates (starts at 66 ends at 70) or if it continues beyond one or both?

It could be any number the State puts forward at AASHTO. 

Consider I-196 and I-696 in Michigan, I-355 in Illinois, and I-275 in Tennessee, all of which connect two Interstates with different numbers. I don't count I-390 in New York for these purposes because its southern end was not originally at an Interstate.

[GaryV]

Consider I-196 and I-696 in Michigan

It has been mentioned here several times before. I-196 was originally planned as a spur off I-96, going from Grand Rapids to Muskegon. It was decided that, when what is now I-196 hadn't yet been built, it made no sense to have a freeway change numbers on the east side of GR. So the GR to Muskegon portion was renumbered as I-96, and when the freeway from GR to Benton Harbor was eventually build in stages, I-196 was applied to it.

[Henry]

Ideally, the first available digit of a 3di within a state should start with a 1 (for spurs) or 2 (for bypass loops), but of course it does not work that way. Several factors come into play here, such as no duplication and/or the refusal to renumber an existing state route (such as CA).

[Rothman]

So if a road is going from say I-66 north to I-70, could it be either even or odd?  Would it depend if the road ends at the interstates (starts at 66 ends at 70) or if it continues beyond one or both?

It could be any number the State puts forward at AASHTO. 

Consider I-196 and I-696 in Michigan, I-355 in Illinois, and I-275 in Tennessee, all of which connect two Interstates with different numbers. I don't count I-390 in New York for these purposes because its southern end was not originally at an Interstate.

Ok...now what?

[roadfro]

Missed this thread originally, but replying to this post:

Nevada: IIRC the usage of I-515 and I-580 (starting with their designations in the 1970s and eventual in-field signings in the 1990s and 2010s respectively) fits in with their state route highway numbering system for some reason.

Urban arterial routes in Nevada start with 5 or 6 and are allocated in blocks by county. Amusingly, though, 515 is a Carson number and 580 is a Clark County number. There's obviously no other way to comply with Interstate numbering, though.

This implies that if I-11 ever gets an auxiliary route in Southern Nevada, it will be I-611, since that's a Clark County number that currently isn't in use.

While Nevada's primary (100-400 series) and tertiary (700-800 series) routes are assigned in ascending order by alphabetical order of counties, the urban routes (500-600 series) were mostly assigned in order by city alphabetically. This was a byproduct of the federal aid highway system numbering that was implemented during the 1976 renumbering, and included many non-state owned/maintained facilities in the numbering. The low 500's were a couple numbers assigned in Boulder City (also in Clark County), and then the 510's started in Carson City and 535 was assigned in Elko. Then come the bulk of Clark County's urban routes Las Vegas Valley routes semi-alphabetically—Henderson got E/W routes in the 560's, then E/W routes in Las Vegas (and North Las Vegas) were assigned in the beginning at the 570s in the north increasing southward into the 590's in unincorporated Clark County, then N/S routes generally from west to east (spanning multiple jurisdictions) from 595 to 612. Reno and Sparks got the 640s through 660s (numbers weren't nearly as orderly with these cities).

In the federal aid system, FAU 611 was likely assigned to some north-south minor arterial between Lamb Blvd & Nellis Blvd (Mountain Vista St in Henderson, IIRC). Since the federal aid system numbering was abandoned (1991?), NDOT has, thus far, to my knowledge, not reused any numbers from that system for any new state routes created since then—they've just added new numbers within the old scheme within the gaps left for expansion in 1976. Therefore, an I-11 auxiliary route numbered 611 seems unlikely.

[SkyPesos]

3.  If a 3 digit interstate ends at another interstate it must be even, correct, EVEN if that interstate is not an offshoot of 2 digit (Fictionally 387 starts at 87 and ends at 684)?

Depends on state and AASHTO's mood at the time of number assignment, as I don't see a consistent pattern (like why MO 370 get rejected for an interstate of that number while I-540 in NC was approved even though both routes serve as bypasses for their parent route). Though I think more odd 3di numbers should be used in that case because every state has 5 odd 3di for a 2di, but only 4 even 3di, so theoretically it's more common to run out of even 3di in a state than to run out of odds.

[mrsman]

3.  If a 3 digit interstate ends at another interstate it must be even, correct, EVEN if that interstate is not an offshoot of 2 digit (Fictionally 387 starts at 87 and ends at 684)?

Depends on state and AASHTO's mood at the time of number assignment, as I don't see a consistent pattern (like why MO 370 get rejected for an interstate of that number while I-540 in NC was approved even though both routes serve as bypasses for their parent route). Though I think more odd 3di numbers should be used in that case because every state has 5 odd 3di for a 2di, but only 4 even 3di, so theoretically it's more common to run out of even 3di in a state than to run out of odds.

Ideally, odd 3dis for spurs or 3dis that end at a 2di on one end only. 

Ideally, even 3dis for 3dis that connect one 2di to another. This would include, but not limited to, many bypasses and beltways.

I would also say that these should not be hard and fast rules, but guidelines.  A roadway that is an even 3di since it connects a 2di to another 2di, but is later extended further on one side so that it now acts as a spur, should retain its even 2di and not be renumbered.  I see no valid purpose for a renumbering and all the extra confusion and signage changes required.

Also, it seems like lower numbered 3dis: 1xx, 2xx get used more than higher numbered 3dis: 8xx, 9xx.  Does not appear to be any rhyme or reason to it, other than perhaps wanting to assign the numbers in some sort of order. 

[vdeane]

Depends on state and AASHTO's mood at the time of number assignment, as I don't see a consistent pattern (like why MO 370 get rejected for an interstate of that number while I-540 in NC was approved even though both routes serve as bypasses for their parent route). Though I think more odd 3di numbers should be used in that case because every state has 5 odd 3di for a 2di, but only 4 even 3di, so theoretically it's more common to run out of even 3di in a state than to run out of odds.

This is why I do no understand the rationale for keeping I-894 around Milwaukee.  They say "the number communicates that it's a bypass of downtown", but that is far from universally true, even in the Midwest.  A sign that says "Downtown Bypass - Follow [I-41] North to [I-94] West" would work better for that and reduce numbering clutter.  Still, I can explain that until I'm blue in the face, but they won't budge, probably because I'm a "meddling northeasterner".

[GaryV]

Also, it seems like lower numbered 3dis: 1xx, 2xx get used more than higher numbered 3dis: 8xx, 9xx.  Does not appear to be any rhyme or reason to it, other than perhaps wanting to assign the numbers in some sort of order. 

That is the rhyme and reason of it. They are generally assigned numerically from W to E and from S to N, just like mile markers. Some numbers fall in between if they come at a later time.