Hm, alright then. I'll go change it back. My reasoning for it being trunk are as follows:
* Residents of Blue Diamond can use SR 159 to get to the west-central portions of the valley quickly. The next-fastest route would be SR 160, then either Durango or I-15 to CC 215.
* Residents of the valley who want to visit Red Rock Canyon basically have to use this road eventually, since it's literally the only road connecting it to Red Rock.