According to Federal Highway Administration (FHA) vehicle safety standards, turn signals for motorcycles must be placed so that the "direction is coincident with the axis of reference." In more simple terms, it means they must be aligned horizontally to the road.
A reflective lens must be located behind the turn signal to enhance its luminescence and make the motorcycle more conspicuous.
Turn Signal Size
Each turn signal (left and right) on a motorcycle must be at least 3.5 square inches (2,258 square mm). If a multiple compartment lamp or multiple lamps are used, each turn signal must be at least 3.41 square inches (22 square cm).
Signal Before Turning
Motorcyclists, like all motorists, must single their intention to turn with adequate time for others on the road to see the signal. It is illegal to change lanes on a highway or make a turn without first signaling your intention to do so. Unlike other vehicles, motorcycle turn signals are turned on and off manually.
Turn Signal Requirements
FHA standards are applicable on all interstate highways, and most states have adopted the FHA guidelines for state and other roadways. This is not true in all states, however. Texas, for example, doesn't require motorcycles to have turn signals. It is advisable, nonetheless, to have turn signals on all motorcycles for safety and legal considerations.
Brightness and Blinking
Turn signals must meet the same candlepower requirements for Class A turn signals on cars. The easiest way to ensure a turn signal light conforms to law is to look at the lens. It should have SAE (Society of Automotive Engineers) or DOT (Department of Transportation), or both, stamped onto the glass lens, which signifies that it conforms with FHA and SAE guidelines. The turn signal must also flash to meet the law's requirements. Headlight manufacturers use flashing relays that rely on capacitors to regulate the flow of electricity to the turn signal
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