This subject has been beaten to death on other MC forums (fora? 60+ years since my last Latin class��

.
_----------------------
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-based or. Title 49 USC 30103 (b1) (US Codes) prohibits any state from forbidding a system that conforms to FMVSS 108.
----------------
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.
--------------------
There's also an elaborate section on how the sensor works to insure that the modulation turns off at night.
This refers to changing the brightness of the headlights. Flashing them ( on/off) is a no-no. If your bike has dual headlights both headlights MUST brighten and dim in sync. Alternating them side to die is a BIG no-no. Almost every state has laws defining "wig-wag" lights as a signal for emergency vehicles, and cops don't like people pretending to be cops, no matter how obliquely. The federal rule isn't going to keep you out of handcuffs.