Another excellent, as usual, Legally Speaking with Bob Mionske, Two-by-two, covers the two abreast issue, covering specifically a situation in Wisconsin. Here’s the low-down on Arizona law:
§28-815 B. Persons riding bicycles upon a roadway shall not ride more than two abreast except on paths or parts of roadway set aside for the exclusive use of bicycles.
Note that our statute, unlike the Wisconsin equivalent, does not have any conditions. Wisconsin statute [346.80(3)(a)] .”Persons riding bicycles or electric personal assistive mobility devices upon a roadway may ride 2 abreast if such operation does not impede the normal and reasonable movement of traffic.”
But since Arizona cyclists are subject to all the other bicycling rules that doesn’t really matter.
Other, really hypothetical, arguments claim that the whole two-abreast thing is a grey area because of the stay-right rule:
§28-815 A. A person riding a bicycle on a roadway at less than the normal speed of traffic at the time and place and under the conditions then existing shall ride as close as practicable to the right-hand curb or edge of the roadway, except…
If we stipulate that none of the exceptions applies at the time, the argument goes, that the “left” (the one further from the curb/edge) cyclist is by definition in violation of this rule. I know of no case law on this subject, and I would conjecture that it’s unlikely there ever will be. [UPDATE there are two superior court decisions which reversed lower-court convictions for not one but two left-2-abreast riders for violating Arizona’s FTR law (Far To the Right, also known as AFRAP, as far right as possible) in a narrow lane. This kind of ruling does not create precedent (because it’s not published) but gives a clear direction of things, see the Arizona v. Roberts, and Arizona v. Picsopo orders, linked at azbikelaw.org/take-the-lane ]
As a practical matter, most places where it would matter around here are urban arterials that have been built with narrow, multiple lanes. Because they are narrow; the ride-right rule, §28-815 A does not apply. Because they are multi-laned; the impeding rule, §28-704 C does not apply (and §28-704 A only applies to motor vehicles, not all vehicles).
So, the bottom line is if you would be allowed to impede when riding single-file — e.g. and most often because of a lane that is too narrow to share — then you may also impede two abreast.
Also note that it is always illegal to ride more than two abreast in the roadway, regardless of its impact on impeding, or anything else.
Other gray areas of abreastness: how does one cyclist pass a pair of cyclists riding two abreast? Does riding two abreast in the roadway, abreast of a third cyclist on the shoulder equate to three-abreast? What about a bicycle lane — is a bicycle lane part of the roadway?
It also occurs to me that groups of cyclists often appear from behind to be operating many-abreast when in fact (as seen from above, say) they are no more than two abreast.
As illustrated and noted in the slide above, there are safety advantages for both bicyclists and motorists when two or more bicyclists choose to ride two-abreast. Because it makes the bicyclists more visible to motorists (from any direction) and allows overtaking motorists more time to plan, eliminating abrupt movements that may be unsafe.
This only applies in a federal jurisdiction, e.g. a national park…
36 CFR § 4.30 Bicycles.
(h)Prohibited acts. The following are prohibited:
(4) Operating a bicycle abreast of another bicycle except where authorized by the superintendent.
It’s not clear where this mis-guided rule comes from. Where applicable, bicyclists are required by federal rules to ride single file; this is not a conflict, just pointless, when a lane is too narrow to share bicyclists (any number of bicyclists, including one) may legally, and should, control the lane, while motorists who wish to travel faster may change lanes to pass when safe. See, e.g. the Natchez-Trace Parkway where BMUFL/CLtP signage was added in early 2017 to remind drivers that a bicyclist is entitled to use the entire lane.