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[kurumi]

Exit numbers on US 20/75 eastbound (northbound) are 1, 2, 3, 4: ordinal but maybe close to US 20 mile markers.

US 75 north of US 20 are milepost-based starting at 93 (US 20 turnoff is exit 4 NB, exit 93 SB). That seems odd, since US 75 enters Iowa along with US 20 just a few miles away.

It looks like Iowa kept the mile markers from the earlier US 75 alignment, which entered at Council Bluffs instead of South Sioux City and generally was displaced by I-29 (and moved to Nebraska in 1984), even though the new US 75 freeway was built 20 years later.

[SD Mapman]

Yup, that's correct. Maybe they had already inventoried the mileposts of US 75 before 1984 and didn't want to change all the rest.

Originally, I believe this was planned to be part of a northern beltway of Sioux City (I-429), but SD quashed the idea direct from the governor.

[The Ghostbuster]

Nature preserves along the Nebraska/South Dakota border would have stood in the way of an Interstate 429 northern bypass of Sioux City. How well utilized would a northern bypass have been?

[iowahighways]

Yup, that's correct. Maybe they had already inventoried the mileposts of US 75 before 1984 and didn't want to change all the rest.

Iowa first used mile markers in 1970, so that's basically why.

Nature preserves along the Nebraska/South Dakota border would have stood in the way of an Interstate 429 northern bypass of Sioux City. How well utilized would a northern bypass have been?

I think the I-429 proposal was before the Iowa DOT decided to widen and reconstruct I-29. Now that reconstruction is finished and I-29 traffic can go 65 MPH straight through Sioux City, I don't see I-429 happening anytime soon.

[SD Mapman]

Nature preserves along the Nebraska/South Dakota border would have stood in the way of an Interstate 429 northern bypass of Sioux City. How well utilized would a northern bypass have been?

There's enough of a gap to make it down the bluffs, besides, this was in the 90s when that wasn't as big of an issue. I can't say about the general population, but I would have used the bypass, and I feel like a good amount of N-S traffic would have as well.

I think the I-429 proposal was before the Iowa DOT decided to widen and reconstruct I-29. Now that reconstruction is finished and I-29 traffic can go 65 MPH straight through Sioux City, I don't see I-429 happening anytime soon.

This is the kicker, I-29 through downtown was miserable for years. Now it's no worse than driving through Sioux Falls.