Here are the results for Iowa:

**IA-MN:**

Formerly, IA 91 & MN 91

IA 60 & MN 60

IA 86 & MN 86

IA 4 & MN 4

IA 15 & MN 15

IA 27

IA 139 & MN 139

IA 76 & MN 76

IA 26 & MN 26

**IA-WI:**

IA 9 & WI 82

**IA-IL:**

IA 64 & IL 64

IA 136 & IL 136

IA 92 & IL 92

IA 163

Formerly, IA 2 & IL 9

**IA-MO:**

IA 27 & MO 27

IA 81 & MO 81

IA 202 & MO 202

IA 5 & MO 5

IA 148 & MO 148

**IA-NE:**

IA 2 & NE 2

IA 165 & Abbott Drive

IA 175 & NE 51

**IA-SD:**

IA 3 & SD 50

Formerly, IA 403 & SD 48

IA 10 & SD 46

IA 9 & SD 42

Out of the 24 current state highways in Iowa that cross a border, 16 of them keep their number in the neighboring state (So, 2/3 of them). It makes sense for it to be a fairly high portion of highways, since Iowa has historically put a heavy emphasis on route continuity with neighboring states (To the point where the original establishment of Iowa's highway system in 1920 prioritized route continuity over creating a numbering system).

There's also IA 92 and NE 92, part of the very important multi-state route from Torrington, WY to La Moille, IL.

SD:

SD-MT:

SD 20 to MT S-328

SD-WY:

SD 34 to WY 24 (note, this used to be SD 24 when the systems were first made, but SD extended 34 over it and got rid of the number continuity)

SD-NE:

**SD 71 to NE 71** (note, this is out of the SD grid but was kept as part of MSR 71)

SD 407 to NE 87

SD 391 to NE 27

SD 73 to NE 61

SD 47 to NE 137

SD 43 to NE 11

SD 37 to NE 14

SD 19 to NE 15

SD-IA:

SD 50 to IA 3

SD 48 to IA 403 (formerly)

SD 46 to IA 10

SD 42 to IA 9

SD-MN:

SD 11 to MN 269

SD 34 to MN 30

SD 30 to MN 19

SD 28 to MN 271

SD 22 to MN 68

SD 20 to MN 40

SD 10 to MN 28

SD-ND

**SD 127 to ND 127** (probably ND keeping the number, since this fits SD's numbering system)

SD 25 to ND 18

SD 27 to ND 32

SD 37 to ND 1

SD 45 to ND 3

**SD 1804 to ND 1804** (Dakota River Roads, doesn't really count)

SD 63 to ND 6

SD 65 to ND 31

SD 73 to ND 49

SD 75 to ND 8

SD 79 to ND 22

I seem to be picking up a theme here. States that organize their state highway numbers based on a grid (like Florida, Indiana, and South Dakota) or by numerical ranges based on the county (like Nevada) tend not to have highways that keep their numbers across the state line - which I suppose makes sense, given that those states are all doing their own thing, presumably without regard to what numbers the neighboring states have given the roads. In your opinion, does my assessment seem right?

Yeah seems legit, WY numbers their road by county as well and there's really no carryover there either except for WY/MT 59, which just so happens to be in the "50-69" county in WY (Campbell).