How are 3 digit interstate #'s determined?

[kphoger]

As for determining which odd or which even number to use, doesn't the state suggest a number and then either get it approved or denied?  At least, that's how I think it works.  In which case, it's whatever number someone thinks is a good idea at the time.

[frankenroad]

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.

I'm pretty sure that's how Ohio does it.  I know that 275, 475, and 675 were constructed in that order; and I'm sure 271 is older than 471.  270 is the oldest of the x70s. In reviewing old maps, it looks like 470 was assigned a number before 670, even though some of what is now 670 was built first.

[RG407]

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.

I'm pretty sure that's how Ohio does it.  I know that 275, 475, and 675 were constructed in that order; and I'm sure 271 is older than 471.  270 is the oldest of the x70s. In reviewing old maps, it looks like 470 was assigned a number before 670, even though some of what is now 670 was built first.

This seems to be how Florida has done it.  I believe 110, 175, 375, 195, 295 and 395 were all part of the original plan for Florida interstates.  After not too long came 275.  Plans for 595 came later, and 795 later still.

[Road Hog]

Arkansas' 2dis and 3dis seem set, so ARDOT is using the midrange 4xx through 6xx state numbers as placeholders for future extensions of major roads. They've already used 3xx as replacement numbers for former US highways subsumed by parallel freeways (namely 365 and 367).

[OCGuy81]

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.

Wisconsin has I-894 and I-794. No other routes were planned.

That is an interesting one.  I always wondered if they skipped 294 because Illinois had already used it not too far south of the Milwaukee metro area.

MAYBE they thought of 394, 494 and 694 in Minnesota, but the Twin Cities are a long way from 894 and 794.  Plus, it wouldn't explain why 194 and 594 were skipped.

[MATraveler128]

One I don't understand is I-985 in Georgia. Obviously I-185 is already taken as a spur to Columbus, but I've always wondered why I-385 and I-585 were skipped. Maybe because of confusion with the ones already in South Carolina. If that's the case they also skipped I-785 which was open as well.

[wanderer2575]

I mentioned before in a similar thread that an equally interesting question is how the "mainline" number part of a 3di is selected when the route has a terminus/termini at more than one.  For example, I-359's southern terminus in Tuscaloosa AL is at the I-20/I-59 duplex.  Was it a random pick between I-359 and I-320, or was there some rationale involved?  Is it because I-359 is signed as north-south?  Okay, then how about I-280 in Ohio, with a northern terminus at I-75 and a southern terminus at I-80/I-90?  It's also signed north-south, so why not I-x75?  If cardinal directions weren't an issue, why not I-x90?

[freebrickproductions]

MAYBE they thought of 394, 494 and 694 in Minnesota, but the Twin Cities are a long way from 894 and 794.  Plus, it wouldn't explain why 194 and 594 were skipped.

Milwaukee is also the eastern part of the state, and towards the eastern end of I-94 in Wisconsin. I wouldn't be too surprised if they wanted to "save" the lower/skipped numbers for potential spurs and bypasses in the parts of the state to the west of Milwaukee.

One I don't understand is I-985 in Georgia. Obviously I-185 is already taken as a spur to Columbus, but I've always wondered why I-385 and I-585 were skipped. Maybe because of confusion with the ones already in South Carolina. If that's the case they also skipped I-785 which was open as well.

I could potentially see I-785 being used for a spur from I-85 to Athens, over what's currently GA 316 and US 29. I could also see I-385 maybe having been saved for a potential Atlanta-area interstate (GA 400?), with I-585 being skipped to avoid potential confusion with I-575.

[Flint1979]

Michigan's go in in order by number or at least that's how it is. I-75 has starting from the south going north I-275, I-375, I-475 and I-675. I-69 doesn't have any, I-94 just has I-194 and I-96 has I-196, I-496 and I-696. I'm not counting I-296 which is unsigned and already has another route number for the entire route.

[ilpt4u]

As for determining which odd or which even number to use, doesn't the state suggest a number and then either get it approved or denied?  At least, that's how I think it works.  In which case, it's whatever number someone thinks is a good idea at the time.

If memory serves ISTHA wanted I-455 for the North-South/now Veterans Memorial Tollway (and early under-construction maps were even printed with 455), but AASHTO approved I-355, feeling the route was more of a suburban spur of I-55 compared to a bypass/loop

[TheHighwayMan3561]

In the case of Minnesota, the numbers are simply the lowest available numbers that aren't already in use by another state highway. I don't know why they chose to stick a 1 in front of former state highways 35 and 94 rather than, say, extending TH 200 over current TH 194 and choosing a completely different number for 135 in order to free up those numbers, but a nice benefit resulting from that is there was no duplication with 3-digit interstates other states. There would have been if 335 was built in Minneapolis and 494 was built in Chicago, though.

The designation change of TH 94 to modern TH 194 predated the creation of TH 200 by about 10 years. 200 didn't come along until about 1969.

[roadman65]

I heard rumors that MO Route 370 is not I-370 because AASHTO rejected that number and wanted I-870 which Missouri disliked. So they kept it a state designation instead.

Don't know if that's true as it was on Wikipedia for the MO 370 article. As we know anyone can change an article on that and there was a user on here who seemed capable of changing facts on roads to fiction just to be a prick as he was a top contributor for Wiki at one time.

[TheStranger]

Some neo-expansionist examples in the current day that are amusing to think about how little (or how much) order was factored in:

- I-490 in Illinois, logical sequence after the existing I-290 even-numbered route.

- The once-proposed I-227 (now I-27W) through Midland, and proposed I-327 (now I-27N!) as part of the Ports to Plains Corridor project/I-27 extension

- I-269 in Memphis, logical sequence, but I recall this was proposed as I-669 years ago.

- For that matter, I-369 was designated first in Texas, then I-169 way further down the state. Example of the geographic numbering coming into play?  For that matter, it's interesting Indiana went with I-469 for the Fort Wayne loop route with 269 still available over there.

- North Carolina's newly concocted I-777 as of this very week!!! when no other odd x77 route even exists at this present time.

[ilpt4u]

Some neo-expansionist examples in the current day that are amusing to think about how little (or how much) order was factored in:

- I-490 in Illinois, logical sequence after the existing I-290 even-numbered route.

I-490 for the ORD Ring Tollway is a bit of a divergence from how Illinois has designated 3dis previously. Other 3dis in IL that only touch their parent once have all received odds, even when connecting 2 (or more) interstates: 155 connects 55 to 74; 355 connects 55 to 80, 88, and 290 directly and 90 indirectly. 490 will obviously only touch 90 once, tho will meet sibling IL 390 also

The previous IL evens all touch the parent twice (tho the second time may not be in Illinois): 255, 270, 280, 290, 294, 474

[TheStranger]

Some neo-expansionist examples in the current day that are amusing to think about how little (or how much) order was factored in:

- I-490 in Illinois, logical sequence after the existing I-290 even-numbered route.

I-490 for the ORD Ring Tollway is a bit of a divergence from how Illinois has designated 3dis previously. Other 3dis in IL that only touch their parent once have all received odds, even when connecting 2 (or more) interstates: 155 connects 55 to 74; 355 connects 55 to 80, 88, and 290 directly and 90 indirectly. 490 will obviously only touch 90 once, tho will meet sibling IL 390 also

The previous IL evens all touch the parent twice (tho the second time may not be in Illinois): 255, 270, 280, 290, 294, 474

Earlier in the thread, someone mentioned that I-355 was originally proposed as I-455, then AASHTO essentially steered them towards the 355 designation. Maybe 490 would have been 590 (factoring in IL 390) in the 1980s, but got its number retained in AASHTO's current lassiez-faire environment?

I did notice that 274 was skipped with Peoria having 474. 

[ilpt4u]

Some neo-expansionist examples in the current day that are amusing to think about how little (or how much) order was factored in:

- I-490 in Illinois, logical sequence after the existing I-290 even-numbered route.

I-490 for the ORD Ring Tollway is a bit of a divergence from how Illinois has designated 3dis previously. Other 3dis in IL that only touch their parent once have all received odds, even when connecting 2 (or more) interstates: 155 connects 55 to 74; 355 connects 55 to 80, 88, and 290 directly and 90 indirectly. 490 will obviously only touch 90 once, tho will meet sibling IL 390 also

The previous IL evens all touch the parent twice (tho the second time may not be in Illinois): 255, 270, 280, 290, 294, 474

Earlier in the thread, someone mentioned that I-355 was originally proposed as I-455, then AASHTO essentially steered them towards the 355 designation. Maybe 490 would have been 590 (factoring in IL 390) in the 1980s, but got its number retained in AASHTO's current lassiez-faire environment?

I did notice that 274 was skipped with Peoria having 474. 

That was me on the 455/355 thing

274 had something proposed in the Quad Cities, if I remember. Maybe what became I-74 downtown?

[roadman65]

How was I-555 picked in Arkansas? Better yet I-540? Does AR like the number 5?

[jt4]

How was I-555 picked in Arkansas? Better yet I-540? Does AR like the number 5?

Looks like 155, 355, 140, and 340 were taken.

[Quillz]

California's use of the x05 auxiliaries is interesting. 205 and 605 are spurs, not loops. Their placement also feels reversed, like 205 should have been in SoCal and 605 farther north. Then you have 105, 505, 905. Again, feels like 505 and 905 should have been swapped, but then it's also curious as to why 305 and 705 were skipped. 305 does exist unsigned, but I've never read any reasoning as to why 705 was never used.

I say "swapped" because it seems the general assumption within the state is numbers progress from west to east. You can see this with 280 and 680; 380 and 580. 780 and 880 don't quite fit but were slotted in later, although the short-lived 480 was west of both. Then in SoCal, you can see 110 west of 710, 405 generally west of 605, etc. 110 and 710 make sense because they were more or less conversions of the original CA-7 and CA-11, so the numbers kind of fit. But otherwise seems some of the outlier numbers like 905 were just sort of picked at random.

[roadman65]

How was I-555 picked in Arkansas? Better yet I-540? Does AR like the number 5?

Looks like 155, 355, 140, and 340 were taken.

I-155 and I-355? Not in Arkansas.

[ilpt4u]

How was I-555 picked in Arkansas? Better yet I-540? Does AR like the number 5?

Looks like 155, 355, 140, and 340 were taken.

I-155 and I-355? Not in Arkansas.

I-155 is nearby, going from the SEMO Bootheel into NW Tennessee, so I think that is a good enough reason to DQ 155, especially in NE Arkansas. 355? Yea, where is the nearest 355, other than Chicagoland?

[hotdogPi]

355? Yea, where is the nearest 355, other than Chicagoland?

Near Texarkana, as a state route.

[TheStranger]

California's use of the x05 auxiliaries is interesting. 205 and 605 are spurs, not loops. Their placement also feels reversed, like 205 should have been in SoCal and 605 farther north. Then you have 105, 505, 905. Again, feels like 505 and 905 should have been swapped, but then it's also curious as to why 305 and 705 were skipped. 305 does exist unsigned, but I've never read any reasoning as to why 705 was never used.

I say "swapped" because it seems the general assumption within the state is numbers progress from west to east. You can see this with 280 and 680; 380 and 580. 780 and 880 don't quite fit but were slotted in later, although the short-lived 480 was west of both. Then in SoCal, you can see 110 west of 710, 405 generally west of 605, etc. 110 and 710 make sense because they were more or less conversions of the original CA-7 and CA-11, so the numbers kind of fit. But otherwise seems some of the outlier numbers like 905 were just sort of picked at random.

I mentioned it earlier in the thread, but the x05 placement seems to be very specifically north-to-south:

https://www.aaroads.com/forum/index.php?msg=2917313

205 (created after the 1958 reroute of I-5 away from Route 99) - Altamont to Tracy
405 (original) - Sylmar to Irvine
605 (original, but extended) - original extent was Seal Beach to El Monte, then designation extended to Duarte/Irwindale
805 (original) - San Diego bypass, Sorrento Valley to San Ysidro via Mission Valley

That might explain why 905 was used right at the border, rather than 705.  Both 105s were/are in SoCal but designated after the 1960s began; 505 is likely derived from the formerly planned 5W designation (note that 580 was not numbered 180 (conflicted with CA 180) or 380 when 5W was de-designated) and 305 was the lowest number x05 available in 1982.

---

For the x80s, the originally planned routes prior to 1964 progressed west to east:
280 would have been the westernmost, had the Route 1/19th Avenue bypass been built between Font Boulevard and Lake Street in San Francisco.
480 was next in the sequence and was originally planned to start at 280/101 in the Presidio.  (The built portion eventually had a western terminus at Broadway and Battery)
680 directly began east of 280 in San Jose (originally at the modern 880/101 junction, and now at the stack interchange east of downtown)
880 was a Sacramento bypass (now I-80)
Interestingly, none of the odd numbered x80 designations existed or were planned pre-1964, since 580 was signed in the field was 5W 1962-1964.

[Quillz]

That's kind of weird, because it seems interstates in general try to put lower numbers in the south and west, and doing it that way also would have kind of fit into the state highway grid, where there was an attempt to have lower numbers as you went west. I guess it fits how they did it, just feels like a reversal.

Although it's interesting that using 905 near the border was in spirit of the original 1934 numbering scheme, which often used larger numbers near the border. 94, 96, 98 were numbered that way. Original SR-95 followed that logic. So I can see it, that does make some sense.

[bluecountry]

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)?