^ Because it would be easier to start over with a new corridor rather than try to fix the existing north-south section of US 12, particularly in regards to access management?
That's not what I want to know. I want to know, why is ANY work on US-12 necessary?
US-12/Hwy 67 has AVERAGE daily traffic ranging from 10,000 to 15,000 vpd (via
https://wisdot.maps.arcgis.com/apps/webappviewer/index.html). It's high enough that WisDOT put a permanent traffic counter in to measure that traffic. That's well within the range justifying 4 lanes.
This area has a high number of summer properties and associated tourist traffic. So frequently, that traffic load is considerably higher. Woe be the poor sucker who has to cross US-12/Hwy 67 during those times. On one occasion. I was trying to follow County ES and was held up for a solid 10 minutes trying to cross US-12/Hwy 67.
WisDOT has a few options to address it. The do-nothing option is becoming increasingly untenable.
WisDOT could make a 4-lane boulevard and put in a bunch of traffic lights at the bigger intersections. Given the limited right-of-way and current development near the highway, that's an expensive undertaking. It also slows traffic and takes away from the rural, scenic character of the area.
Much of the traffic choking this stretch is not local tourist traffic - it's regional traffic trying to go elsewhere - for instance, Whitewater. There's a large UW campus there. There's also considerable manufacturing presence there, including Generac.
The so-called "Corner Cut" takes this regional traffic off the existing US-12/Hwy 67. Now the traffic on that road is primarily people with a local destination. Take 4,000 to 5,000 vpd off that stretch (a not unreasonable assumption), and you've fixed a lot of problems without having to widen the existing road. A Super-2 highway on 4-lane right-of-way (much like the Whitewater bypass or the Hwy 11 Janesville bypass) will solve a lot of traffic headaches in the area for decades, and make things easier later on if a widening is eventually necessary.
It's costs are a drop in the bucket compared with the costs of widening the Beltline or widening I-94 in Milwaukee. 10 miles of 2-lane highway on new ROW in a rural area isn't going to bust WisDOT's budget, especially considering some of the other area projects they've built (Hwy 26, Burlington bypass, etc). Ignoring this isn't going to magically free up cash that makes a dent in the Beltline's traffic issues.