I find it odd that in 30 years of North American touring I have never encountered such an assessment of the TransAm before.
Yes, there are segments of the route with higher than preferable traffic volumes, but those are a clear minority.
I know the TransAm well, I have 100,000 miles of riding, and I know the West as well as anyone here.
I suspect a number of things at play - overall fatigue, unease with the extreme empty spaces,
and perhaps, a hyperfocus on each and every car that passed as they became more central for you.
Kansas has many, many miles of very low traffic.
Hwy 96 in west Kansas has low to very low AADTs - 500 to 1000 most of the way.
The county roads thru the Flint Hills and Cassoday have almost no traffic.
US 54 is moderately busy, but from Toronto to Chanute to Girard is practically empty.
I will agree that there are some seriously sucky sections in Colorado.
The level of development in the Colorado Rockies over the past 25 years -
has made it extremely difficult to find lightly traveled roads there any more.
Also, Colorado state highways have some of the least shoulders - Denver getting all the $$$.
Wyoming does have fewer paved road options, but many stretches have low traffic.
Hwy 230 from the Colorado border to Saratoga has light traffic as does the old road thru Sinclair.
US 287 north of Rawlins is pretty gnarly, but west of Muddy Gap to Lander is open road.
Of course, as you get closer to Yellowstone, the traffic does pick up - you and the other 4 million visitors.
And Oregon? Most of the TransAm in eastern Oregon uses lightly traveled roads.
Hwy 86 from the Idaho border to Baker City has very little traffic.
Hwy 7 from Baker City to Austin Jct. has very little traffic.
US 26 has light traffic all the way to Prineville.
You may have faced unusually high traffic number on a day or two - but not for a month.
I know - because I have been on these roads, multiple times.
Yes, cars in the West usually go fast - it's 50 miles to the county seat and 200 miles to do any shopping.
But I find your assessment to be at odds with my experience and the experiences of many, many others.
https://www.ksdot.org/Assets/wwwksdotorg/bureaus/burTransPlan/maps/CountMaps/Districts/countmap2017.pdfhttps://apps.wyoroad.info/itsm/map.htmlhttps://www.oregon.gov/ODOT/Data/Documents/Flow_Map_2016.pdf