What do the states have to say about this?
Well, a whole lot. Some of it pretty good, a lot of it bad and a lot of it references 16CFR1512 either by direct reference or by cutting and pasting the language from various generations. Lets take a look at a some examples.
Oklahoma vehicle code §47-1-104 defines a bicycle thusly — Bicycle, electric-assisted bicycle, and motorized bicycle. A. A bicycle is a device upon which any person or persons may ride, propelled solely by human power through a belt, chain, or gears, and having two or more wheels, excluding mopeds. (You can click on the state outline above or here to get the direct reference)
Now just across the border
Texas Vehicle code 541.201 defines a bicycle thusly —
(2) “Bicycle” means a device, excluding a moped, that is capable of being ridden solely using human power and has either:
(A) two tandem wheels at least one of which is more than 14 inches in diameter;
(B) three wheels, two of which are in parallel, and at least one of the three wheels is more than 14 inches in diameter; or
(C) any number of wheels and adaptive technology that allows the device to be ridden by a person with a disability.
As before you can click the state or here for the direct reference to the law
Ok that wasn’t a great definition, are there worse examples?
New Jersey transportation code NJ Rev Stat § 39:4-14.5 (2020) defines a bicycle thusly — As used in this act “bicycle” means any two-wheeled vehicle having a rear drive wheel which is solely human-powered and having a seat height of 25 inches or greater when the seat is in the lowest adjustable position