Greater Phoenix driver here, and I can say 'Well... Sort of."
The central Phoenix area (aka downtown) is pretty jammed during business days during the start of rush hour (~5 am or thereabouts) to early evening (~8-9 pm) most days, unless there's a sports event or something else going on. then it can be moderate traffic until midnight.
The outer cites (Tempe, Socttsdale, Glendale, Mesa, etc.) each have a similar downtown area with similar traffic density issues. (Mesa is especially egregious because they plonked a bloody rail line down the middle of it for our mass transit system.)
During the day time outside of rush hour, the feeder streets are largely empty, and the main thoroughfares have light to moderate traffic on them.
The freeways combine the 'fish in a barrel' density of California roads, the 'devil may care' attitude of New York drivers, and the 'I'm too old to be driving but I'll be safe in the leftmost lane' insanity of seasonal drivers and tourists. I don't think a machine would stand a chance in that zoo.
The main strength we have is that our streets are laid out in a mostly grid arrangement, with a few exceptions for topology and the odd railroad line that existed before the city became a sprawling metropolis.
I know that Waymo's main competitor, Uber, has been driving their little fleet of autonomous cars in a set pattern around the Tempe area. I've not really been paying attention, but the code that runs them seems to be competent enough, and the drivers appear to be largely polite or at least disinterested around them.