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Official U.S. Code of Federal Regulations Governing Motorcycle Headlight Modulators

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Printer Friendly Version: Print this smaller version of the regulation page to keep with you on your motorcycle in case you get stopped by the Police.

Headlight modulators are now legal in Canada; here are the Canadian regulations in English or French.

This Federal law supersedes all state laws and makes motorcycle headlight modulators legal in all 50 states.  FMVSS 108 (Federal Motor Vehicle Safety Standards) (49 CFR Part 571.108 S7.9.4) allows motorcycle headlight modulation systems all 50 states provided they comply with the standards set forth in this section.  

Title 49 USC 30103 (b1) (US Codes) prohibits any state from forbidding a system that conforms to FMVSS 108 (see copy below).  Here is a web page with all of Part 571 and all of the subsections; they've now broken it up by section to make it easier to read.  The Legal Information Institute at Cornell University also has a nice website where you can read the various sections of the entire Title 49 USC.

Code of Federal Regulations
Title 49, Volume 5, Parts 400 to 999
Revised as of October 1, 2000
From the U.S. Government Printing Office via GPO Access [CITE: 49CFR571.108] [Page 236-307]


Subpart B--Federal Motor Vehicle Safety Standards
Sec. 571.108 Standard No. 108;

Lamps, reflective devices, and associated equipment.

S7.9.4 Motorcycle headlamp modulation system.

S7.9.4.1 A headlamp on a motorcycle may be wired to modulate either the upper beam or the lower beam from its maximum intensity to a lesser intensity, provided that:

(a) The rate of modulation shall be 240 <plus-minus> 40 cycles per minute.

(b) The headlamp shall be operated at maximum power for 50 to 70 percent of each cycle.

(c) The lowest intensity at any test point shall be not less than 17 percent of the maximum intensity measured at the same point.

(d) The modulator switch shall be wired in the power lead of the beam filament being modulated and not in the ground side of the circuit.

(e) Means shall be provided so that both the lower beam and upper beam remain operable in the event of a modulator failure.

(f) The system shall include a sensor mounted with the axis of its sensing element perpendicular to a horizontal plane. Headlamp modulation shall cease whenever the level of light emitted by a tungsten filament light operating at 3000 deg. Kelvin is either less than 270 lux (25 foot-candles) of direct light for upward pointing sensors or less than 60 lux (5.6 foot-candles) of reflected light for downward pointing sensors. The light is measured by a silicon cell type light meter that is located at the sensor and pointing in the same direction as the sensor. A Kodak Gray Card (Kodak R-27) is placed at ground level to simulate the road surface in testing downward pointing sensors.

(g) When tested in accordance with the test profile shown in Figure 9, the voltage drop across the modulator when the lamp is on at all test conditions for 12 volt systems and 6 volt systems shall not be greater than .45 volt. The modulator shall meet all the provisions of the standard after completion of the test profile shown in Figure 9.

(h) Means shall be provided so that both the lower and upper beam function at design voltage when the headlamp control switch is in either the lower or upper beam position when the modulator is off.

S7.9.4.2(a) Each motorcycle headlamp modulator not intended as original equipment, or its container, shall be labeled with the maximum wattage, and the minimum wattage appropriate for its use. Additionally, each such modulator shall comply with S7.9.4.1 (a) through (g) when connected to a headlamp of the maximum rated power and a headlamp of the minimum rated power, and shall provide means so that the modulated beam functions at design voltage when the modulator is off.

(b) Instructions, with a diagram, shall be provided for mounting the light sensor including location on the motorcycle, distance above the road surface, and orientation with respect to the light.

Here is a copy of the relevant section of Title 49, United States Code, Chapter 301 Motor Vehicle Safety.  This law prohibits states from forbidding a system that complies with FMVSS 108.  The full document can be found at this link.


Sec. 30101. Purpose and policy. 
Sec. 30102. Definitions. 
Sec. 30103. Relationship to other laws. 

UNIFORMITY OF REGULATIONS The Secretary of Transportation may not prescribe a safety regulation related to a motor vehicle subject to subchapter II of chapter 105 of this title that differs from a motor vehicle safety standard prescribed under this chapter. However, the Secretary may prescribe, for a motor vehicle operated by a carrier subject to subchapter II of chapter 105, a safety regulation that imposes a higher standard of performance after manufacture than that required by an applicable standard in effect at the time of manufacture.


When a motor vehicle safety standard is in effect under this chapter, a State or a political subdivision of a State may prescribe or continue in effect a standard applicable to the same aspect of performance of a motor vehicle or motor vehicle equipment only if the standard is identical to the standard prescribed under this chapter. However, the United States Government, a State, or a political subdivision of a State may prescribe a standard for a motor vehicle or motor vehicle equipment obtained for its own use that imposes a higher performance requirement than that required by the otherwise applicable standard under this chapter.

A State may enforce a standard that is identical to a standard prescribed under this chapter.

ANTITRUST LAWS This chapter does not exempt from the antitrust laws conduct that is unlawful under those laws; or prohibit under the antitrust laws conduct that is lawful under those laws.

WARRANTY OBLIGATIONS AND ADDITIONAL LEGAL RIGHTS AND REMEDIES Sections 30117(b), 30118-30121, 30166(f), and 30167(a) and (b) of this title do not establish or affect a warranty obligation under a law of the United States or a State. A remedy under those sections and sections 30161 and 30162 of this title is in addition to other rights and remedies under other laws of the United States or a State.

COMMON LAW LIABILITY Compliance with a motor vehicle safety standard prescribed under this chapter does not exempt a person from liability at common law.

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