There are two sets of laws in most states, as there is in the federal government. The first set of laws are passed by the legislature often referred to as the Title, or Code. The second set of laws are administrative regulations, and although not passed by the legislature still have the force of law. Although criminal charges usually cannot be brought with violations to administrative regulations, there may still be stiff fines.
In Maryland, for example, the state law says that motorcycles are not required to have electric turn signals.
Maryland State Law
Consequently, a police officer in Maryland cannot legally issue you a citation for not having electric turn signals on your motorcycle.
Turning quickly to the Maryland COMAR (Code of Maryland Regulations) it states..."Reject any vehicle if: (2) Turn signals do not properly indicate right and left when so switched."
Located at...COMAR for lights on motorcycles
With regard to motorcycle lights, the Handbook for Motorcycle Inspections in the state of Maryland states at 11.14.03.08 #10, "Reject any vehicle if: Any lamp, lens, or reflector is not an approved type."
It does not state anywhere in the Handbook or anywhere in the COMAR where this approval can be found. The Administrator of Maryland DOT is provided the authority later on in the COMAR to approve all equipment for motor vehicles in Maryland. Unfortunately, once again the COMAR fails to state where this approval can be found, or how it will be determined. One would assume that the Administrator keeps a list of approved equipment including lamps, lenses, and reflectors.
However, that isn't the case. Instead the Maryland State Police have informed all the Inspection Stations to look for the raised DOT letters, which they claim is approval by the US DOT. They are wrong. The US DOT does not have the authority to approve vehicles or equipment. The US DOT, under the NHTSA, sets standards for new and imported vehicles. With regard to lights and lighting equipment they determine candle power (lumen) and color of lenses to be used. They set standards on where this equipment will be located, generally.
The raised "DOT" letters are placed on equipment by the manufacturer attesting to the manufacturer's certification that their equipment is in conformity with US law and the CFR (Code of Federal Regulations), it is not an approval by the US DOT.
Here is a letter from the Chief Counsel of the NHTSA stating that DOT raised letters do not provide approval by US DOT, on page 2, the highlighted part...US DOT Letter
What's more, there is no requirement for cars, trucks or buses in Maryland to be rejected by the inspector if "lamps, lenses or reflectors are not approved." This would seem to be discriminatory against motorcyclists, for no good reason.