Having just completed a road trip from San Diego through Denver, I thought I'd add my two centavos about the rest stops along I-15 and I-70.
- California, I-15: What rest stops? There are a few spots to pull over, but nothing really available for the road-weary public. The closest I could drum up were pull-over spots along the steep uphill portions for trucks to cool down.
- Arizona, I-15: The scenery makes the entire sliver of I-15 through Arizona a rest stop. Unfortunately, there are no areas to pull off and perform basic bodily functions in private.
- Nevada, I-15: The first thing you notice is just how damn nice and pothole-free the roads are, especially after crossing the state line from California. Again, there are no rest stops, but you're in the midst of a huge open flat expanse, so you could relieve yourself and have folks 10km away watching you. You could always stop in Las Vegas so you can lose money as well as bodily waste, but that's your call. I usually skip Vegas.
- Utah, I-15: More expansive wastelands, but they actually have decent rest stops, especially along...
- Utah, I-70: Plenty of rest stops, and very nicely constructed. No food is available, but they usually have soda machines that actually have sodas in them. The rest rooms are clean and cool in the hot desert sun, the scenery is beautiful in a minimalist fashion, and they are far enough off the highway where the truckers barreling through don't disturb you. I almost always drive straight through to Utah before taking a break (if I need one).
- Colorado, I-70: Roads are a bit worse for wear and the rest stops are just pull-offs for sleeping truckers, but the scenery is grand.
Nothing I've seen out in the West can compare to the stops along the toll roads and highways in New York, which have restaurants, gas stations and plenty of spots to catch a snooze.