Are roadway speed limits enforced with zero tolerance? If not why the difference?

In Arizona, US - speeders are 'given' an 11mph (17kph) over tolerance.

"Why are speeders allowed an 11 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 10 mph over the posted speed limit, the probability of being involved in a crash doubles. However, at 20 mph 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."

Must cyclists have a speedometer? It can be very hard to stay below 20kph without one.