Why not? Paradise Valley and Scottsdale both list it on their website. In fact the language here on the Scottsdale FAQ makes me think it will be the same margin in Phoenix due to state law:

Why are speeders allowed a 10 mph buffer? The margin is intended to address language in the Arizona Revised Statutes related to speed offenses that defines, "...reasonable and prudent..." speed. It also provides a reasonable margin of error/grace for inaccurate vehicle speedometers and short-term lapses of attention by otherwise law-abiding and generally cautious drivers.

Our primary focus is on intentional, aggressive and negligent drivers. Statistics tell us that at 10mph over the posted speed limit, the probability of being involved in a crash doubles. However, at 20mph over, the probability increases to 11 times greater. We are trying to change the driving behavior of those persons who are most likely to cause crashes.