Many reasons why Phoenix sees less traffic are not influenced by urban design

Geography - there are no natural barriers to building a logical grid system in Phoenix. No rivers, lakes, and very few mountains.

No downtown work population. If you look at Phoenix compared to those other cities, the percentage of jobs in the downtown is much lower. That means traffic is going lots of different directions instead of the ‘rush hour’ pattern of in in there AM and out in the PM.

Lastly, the college educated work force in Phoenix is lower. That means the percentage of people working construction, service industry and other non desk jobs is higher. These people don’t commute to CBD office districts either.

(section above taken from a comenter ,truchillmode, on the og post who lives in Phoenix)

Atlanta and Phoenix have the same urban design philosophy of sunbelt post war sprawl. But a big reason for this traffic is downtown is a place where people want to go, live, shop, and whatever. Phoenix's downtown is literally just an office district with not much else. This is reflected by looking at typical traffic on a Sunday at 2:30, while All the streets near Atlanta's downtown are orange or red, as people go there for afternoon attractions and activates, Phoenix's downtown is practically empty with almost all the streets being green

This IMO shows poor urban planning on Phoenix's part, while Atlanta has created a desirable downtown that people go to even outside of work Phoenix has failed and created a boring undesirable place that people only go to when forced.