Using only the even numbers

[kurumi]

Random interstate doodling:

43 states in the continental U.S. are served by even-numbered two-digit interstates (all except AK, HI, DE, RI, VT, NH, ME).

You can visit all these states by using even 2dis only (some U-turns or long detours may be involved; and we don't worry about odd 2dis overlapping the even 2dis.) For example, you can get from LA to Chicago using 10, 20, 30, 40, 44, 70, 74, 80, and 88.

You can't do this with odd numbers: there are "canyons" between 15 and 25, for example, where no odd interstates bridge the gap. You can get around the New England states, and almost anywhere east of I-65; but west of that the options dry up.

The linchpin of the even-only system: I-44, which bridges the "40 and under" group (multiple connections including I-30 and I-26) and the "64 and over" group. From FL to WA, or AZ to CT, you're using I-44.

If I-69 is completed, it will extend the north-south mesh to the I-35 corridor, getting you further west. One of the proposals for I-27 Ports to Plains would connect I-25 at Raton, NM to I-35 at Laredo. But I can't see a new north-south interstate going diagonally from I-15 to I-25. The least Fritzy of those ideas (and it's still very fictional) might be Billings to Great Falls as an I-25 extension.

(Also, even though the southernmost stretch of I-15 in San Diego is a state route, I think we'd consider that still usable for the 5/15 connection.)

[CoreySamson]

Also, there are some interstates that are "islands", where a traveler would not be able to access any other "like" interstates from it (e.g I-27 can't access any other odd interstates). I think this would be a complete list of them:

I-2, I-4, I-11, I-14, I-16, I-17, I-19, I-22, I-25, I-27, I-49, I-78, I-99

Some of them are kinda obvious, but I'm honestly surprised to see I-49 and I-78 included here. Of course, in the future we'll likely see I-2, I-11, I-14, I-27, and I-49 off this list.

[ilpt4u]

I-11 is coming off the list very soon, now that AASHTO has approved the I-11 designation inside I-215 into Vegas over I-515 and US 95, which will get it to the Downtown LV interchange with I-15

[ilpt4u]

The linchpin of the even-only system: I-44, which bridges the "40 and under" group (multiple connections including I-30 and I-26) and the "64 and over" group. From FL to WA, or AZ to CT, you're using I-44.

Until relatively recently, I-44 wasn't able to bridge this gap, as its northeastern terminus previously was at I-55 near the Soulard neighborhood in St Louis, south of the PSB. When the Stan Musial Bridge opened and I-70 was routed across it instead of the Poplar Street Bridge, the "orphaned"  bit of former I-70 near The Arch/Between the Stan Span and the PSB ended up designated as an extended I-44 (as opposed to going unnumered or christening a short 3di for the segment). Easy enough to do, as it only tacks on a few eastern end miles, so it didn't change existing I-44 exit numbers or mile markers, just added some new ones at the high end

You can visit all these states by using even 2dis only (some U-turns or long detours may be involved; and we don't worry about odd 2dis overlapping the even 2dis.) For example, you can get from LA to Chicago using 10, 20, 30, 40, 44, 70, 74, 80, and 88.

Also to follow this routing, probably easier to cross the Mississippi via the PSB, which is I-55/64 (So it keeps an Even) as there is no direct I-44 East -> I-70 East ramp for the Missouri Stan Span approach. One would have to exit I-70 WB and re-enter I-70 EB to cross the Stan Span

I-80 to I-88 won't get you into the Chicago City Limits. Would need to use I-90 or I-94 from I-80 to do that

Speaking of I-80/90, you can use your routing to get LA to Boston, thanks to the 80/90 multiplex