IT TOOK THEM SIX YEARS??????
The major traffic-clogging mainline rebuild portion really got underway in 2017. There were a couple interchange and bridge replacements in the years prior to that, but no substantive mainline work until 2017. So, more like 4 years.
WisDOT had it planned for a 4-year build out, but the politicians in Madison pulled political shenanigans before it was to move forward, and caused it to be delayed due to lack of funding. Then they made a show of putting it back on the calendar and funding it to be completed in "only" 6 years.
Also remember they had to maintain traffic, deal with typical Wisconsin weather, and so on.
Yes, ISTHA got their rebuild between Elgin and Rockford done in 2 years. But they didn't replace many bridges. Also, their traffic maintenance, particularly the first year, was so horrible I drove 30+ miles out of my way to avoid using it. It was posted for 45 for 40 miles and was so narrow I nearly lost my mirrors on multiple occasions.
By contrast, I thought WisDOT did a fine job maintaining traffic flow on I-39/90 during construction. It was always at least 2 lanes each way, and the construction zones were never the entire stretch at once - generally a 10 mile section, then a break of 5-10 miles, then maybe another 5-10 miles. We could also maintain at least 60 in the construction zones. Maybe it took 6 years to complete, but it wasn't unusable for those 6 years.
SM-G991U