California
Nevada (PART 1/PART 2/PART 3/PART 4)
Arizona
Utah
Idaho
Montana
Somewhere between Las Vegas and the Los Angeles/San Diego metropolis, there's an exit marked for Zzyzx Road. It is the last alphabetically named place in the United States.
I-15 had two suffixed branches. I-15E (now I-215) was a loop in the Inland Empire, and I-15W (now the western half of I-86) was a spur route in Idaho.