Number switcharoo

[Roadgeekteen]

I-10 took over part of US 90 and I-90 took over much of US 10.

[Some one]

I-70 bypassed/replaced most of US 40 east of Denver and I-40 bypasses/replaced US 70 east of Little Rock

[Scott5114]

Since the Interstate and US grid are flip-flopped, you would expect this to be the case more often than not.

[TheHighwayMan3561]

I-80 bypassed US 20 through Indiana and part of Ohio
I-20 replaced/bypassed US 80 from Texas to Mississippi

[STLmapboy]

US 91 is erased by I-15 until a short stretch in ID.

[Roadgeekteen]

US 91 is erased by I-15 until a short stretch in ID.

But I-91 didn't replace US 15.

[briantroutman]

US 22 → I-78
US 78 → I-22

(at least parts of both pairs)

[ozarkman417]

US 66 (part) -> I-44

[TheHighwayMan3561]

US 66 (part) -> I-44

But US 44 never became part of I-66. That's the other half of the game.

[kurumi]

US 91 is erased by I-15 until a short stretch in ID.

And I-91 replaced CT 15 as the primary New Haven to Hartford route in CT (15 still exists, but you wouldn't use it for the entire trip).

[sparker]

Bit of a stretch, but here goes:

I-55 follows part of (former) US 66.
I-66 follows much of VA 55.

[Konza]

I-35 follows US 69 from Kansas City to Des Moines.  US 35 is concurrent with I-69 between Marion and Muncie, Indiana.

[GenExpwy]

In a somewhat different vein: Manitoba 18 is a continuation of North Dakota 30, and MB 30 is a continuation of ND 18.

[ozarkman417]

This is a bit of a stretch, but: I-40 and US 64, and I-64 and US 40. Only reason I call it a stretch is that US 40 only follows I 64 for 45 miles or so around the St Louis area. If I-64 replaced anything, it would be US 60 and 460.

I-40 follows US 64 from Warner, OK to Conway, AR; which isn't all that long, either.