Not worst ... rudest.
There's a difference.
I still find it hard to believe having driven in almost every state in the country. I can think of several that I think are worse.
But there are quite a few rude drivers here in the valley. I think it has to do with a clash of driving styles. Idaho and Socal. (I read where Californians now outnumber Idahoans here in the Treasure Valley by-the-way)
Native Idahoans for the most part are courteous and respectful when they drive. Travel a little slower by nature. That's the way it was when most of us learned to drive here.
Californians on the other hand had to lean how to drive more aggressively to get to where they wanted to go. That aggressive driving style up here comes across as rude and hostile at times and makes for pissed off aggravated rednecks that then get stupid.
The freeway between Caldwell and Boise can be a battleground during the morning rush hour.