I-84 and I-184 in Boise is easily the busiest here.
per
this article, it's seen an increase from 215,800 vehicle trips/day in 2003 to 326,300 trips/day in 2018 (weekday traffic, both directions).
for me, if there are any more than 4 cars on the road during my commute, I start shaking my fist and yelling "git off my damned road you dang whippersnappers!!"