Speed Limit

Speed limit signs, maximum speed limits that is, are seen all over town, in the city, and in the country. Some types of roads have “statutory” maximum speed limits, and the limit signs need not be posted; for example residential areas have a statutory limit of 25mph.

The sign simply says “SPEED LIMIT” along with the number which must be in multiples of 5mph (due to MUTCD). The speed limit establishes the maximum speed at which a driver is considered to be driving at a reasonable and prudent speed; though slower speeds than posted may be required, depending on conditions. Anything above that speed is considered to be  prima facia evidence of speeding. Arizona’s speed law is §28-701A  and is often referred to generically as the “basic speed law“. Violation of this law is often called “SPEED GREATER THAN REASONABLE AND PRUDENT”, or,  particularly in the event of a collision may alternately be referred to as “FAILURE TO CONTROL SPEED”, on a bail card (the “ticket”)  both referring to the same statute:

28-701. Reasonable and prudent speed; prima facie evidence; exceptions

A. A person shall not drive a vehicle on a highway at a speed greater than is reasonable and prudent under the circumstances, conditions and actual and potential hazards then existing. A person shall control the speed of a vehicle as necessary to avoid colliding with any object, person, vehicle or other conveyance on, entering or adjacent to the highway in compliance with legal requirements and the duty of all persons to exercise reasonable care for the protection of others.

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Minimum Speed Limits

Minimum speed limits may be established “where engineering judgment determines that slow speeds on a highway might impede the normal and reasonable movement of traffic” (MUTCD, though the Arizona law is even more stringent). Where so established, they are signed with a placard, that is they must be used in conjunction with a maximum speed limit sign, not alone (there is also a combo sign). Here is the statute which supports minimums:

 §28-704B. Minimum speed limits… If the director or local authorities within their respective jurisdictions determine on the basis of an engineering and traffic investigation that slow speeds on any part of a highway consistently impede the normal and reasonable movement of traffic, the director or local authority may determine and declare a minimum speed limit below which a person shall not drive a vehicle except when necessary for safe operation or in compliance with law.

Posted minimum speeds are rare. There are NO KNOWN posted minimums on any street in Arizona.
UPDATE: ADOT confirms there are NO minimum speeds anywhere on the the state highways system, which includes both freeways, like I-10, I-40 and rural highways like US60, SR89, etc…  see reply from ADOT in comment below.

Why am I telling you this?

Because, appallingly, some police either intentionally (lie or tell falsehoods) or through poor training insist that maximums are minimums.  Like this Flagstaff PD officer.

Are Bicyclists subject to posted minimums?

Nobody knows, it may be the case, e.g. this is a rule which “by their nature can have no application”. In which case it wouldn’t apply.

Happily, we don’t need to ever find out or test this because as mentioned above, there are no known streets anywhere in Arizona where bicyclists aren’t prohibited (or restricted. Meaning limited access highways) that have established minimums.

Slow Vehicles

Besides bicycles, there are many classes of vehicle permitted on any street in Arizona with limitations as noted. These road users, and bicyclists, are sometimes going to be going slower than the “normal speed of traffic”. This is by design, and is the expected use of these roads. [content and examples moved to new article  slow-by-nature ]

Criminal speeding

In Arizona, the highest maximum speed limit anywhere is 75mph. Typically or only on rural controlled-access “freeway” highways. Rural highways are typeically posted 50 – 65mph depending on the design.

Speeds anywhere > 20mph over posted or statutory limits become the crime of “Excessive Speed”; §28-701.02A., a misdemeanor.

[Fomerly, anything in excess of 85mph was criminal, so e.g. going 11 over a 75mph posted was criminal. The Republican-controlled legislature waterred that down somewhere around 2018 timeframe. See the sad case of Lake Havasu City Republican state representative Paul Mosley’s 2018 excessive speeding.]

The highest posted limit anywhere in the US is 80mph, except for a small area in Texas with 85mph.

SIDEBAR: The way the automobile industry designs and markets cars...

Tesla says its  Tesla Model S P100D in Ludicrous mode (ludicrous is one step higher than insane mode -- this is according to Tesla) is among, if not *the*, fastest production car in the world in terms of acceleration. Its motors develop something like 762 horsepower. Its maximum speed, limited through software, is only 155mph.

But maybe this is just a one off?

The Ford Mustang is available with "Four Engines. One Soul", according to Ford; offering from a low of 300 horsepower up to 526 horsepower. Some or all of them are limited to a top speed of 155mph. 

But it isn't just Ford, there's GM's Camaro and Corvette, and Chrysler's Charger.

But it isn't just "muscle" cars, innocuous sedans, the Accords, Camrys,  Taurus all sport multi-hundred horsepower engines and are capable of extra-legal speed (2016 Camry 2.6L SE "Top speed  -- governor limited -- 112 mph")

Index of Arizona speed laws (#laws)

For reference, here are all the laws in ARS chapter 28 relating to speed:

Article 6	Speed Restrictions
28-701	Reasonable and prudent speed; prima facie evidence; exceptions
28-701.02	Excessive speeds; classification. A.K.A. “Criminal speeding”
28-702	State highway speed limits
28-702.01	Urbanized areas; waste of a finite resource; civil penalties
28-702.03	Maximum speed limit violation on interstate highway system in another state; effect
28-702.04	Maximum speed limit on interstate highways outside urbanized areas; definition
28-703	Alteration of speed limits by local authority
28-703.01	End of speed zone; signs required
28-703.02	Establishing speed on multiple lane highways
28-704	Minimum speed limits; requirement to turn off roadway
28-705	Special speed limitation; motor driven cycles
28-706	Special speed limitations
28-707	Charging speed violation; burden of proof; findings
28-708	Racing on highways; classification; exception; definitions
28-709	Maximum speed limit for large vehicles and vehicles with trailers
28-710	State highway work zone safety; civil penalty; fund

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MUTCD

Section 2B.13 Speed Limit Sign (R2-1)

Section 2B.16 Minimum Speed Limit Plaque (R2-4P)