In Wisconsin, people call the interstates "The I".
Doesn't matter which one - 39, 41*, 43, 90, 94.
They are "The I".
* There is a section of US Route 41 that is designated as an interstate as it has no access points other than on and off ramps. No signalized intersections and limited access. Runs from the Illinois border up to where it hits I-43 in Green Bay.