Pennslvania law as to what MUST be "Registered":

75 Pa.C.S. § 1301 (2011)

§ 1301. Registration and certificate of title required.

(a) Driving unregistered vehicle prohibited. --No person shall drive or move and no owner or motor carrier shall knowingly permit to be driven or moved upon any highway any vehicle which is not registered in this Commonwealth unless the vehicle is exempt from registration

What is Registration:
75 PA C.S. § 102
"Registration." --The authority for a vehicle to operate on a highway as evidenced by the issuance of an identifying card and plate or plates.

What is a "Vehicle" (also under 75 PA C.S. § 102):

"Vehicle." --Every device in, upon or by which any person or property is or may be transported or drawn upon a highway, except devices used exclusively upon rails or tracks. The term does not include a self-propelled wheelchair or an electrical mobility device operated by and designed for the exclusive use of a person with a mobility-related disability.

Notice the definition of "Vehicles" include bicycles, but bicycles are exemped under 75 P.S. § 1302:

75 Pa.C.S. § 1302 (2011)

§ 1302. Vehicles exempt from registration.

The following types of vehicles are exempt from registration:

(7) Any vehicle moved solely by human or animal power.

I did list the whole section, but it contain nothing but exemptions as to vehicles of car dealers, Farming equipment, towing equipment if towed by a licensed vehicle and other items that rarely go on the public road (For example golf carts that CROSS a highway, does not have to have a license, but if it is used elsewhere it must be licensed) Thus in Pennsylvania "Electric Bikes and Trikkes" MUST be licensed and while I am not citing the statute, so must the operator of such vehicles.

My point is that in many states, it is a licensed vehicle operated by a licensed operator or a bicycle (With the Segway as alternative).