Mr. David Robertson

Environmental and Safety Engineering

 Mazda North America Operations

1025 Connecticut Ave NW, Suite 910

Washington, DC 20036

 Dear Mr. Robertson,

This responds to your letter requesting an interpretation of the “steady burning” requirement for parking lamps under Federal Motor Vehicle Safety Standard (FMVSS) No. 108, Lamps, Reflective Devices, and Associated Equipment. You ask how the steady burning requirement for parking lamps applies to an optically combined LED parking/turn signal lamp while the turn signal is activated.1

You describe in your letter an optically combined LED parking/turn signal lamp which contains two discrete LED light sources: one that is amber and one that is white. When the LED lamp operates as a parking lamp only, the amber LED is off and the white LED is steady burning.  However, you ask about the permissibility of two alternative ways in which these LEDs could possibly operate once the turning signal is activated. Under the first alternative (“Alternative A”), the amber and white LEDs flash in an alternating pattern. To an observer of Alternative A, the lamp would alternate its color between amber and white for the duration of the turn signal’s activation, then would return to steady-white. Under the second alternative (“Alternative B”), the white LED turns off for the duration of the turn signal’s activation. To an observer of Alternative B, the LED lamp would alternate between amber and “off” for the

duration of the turn signal’s activation, then would return to steady-white. This interpretation assumes that your lamp’s LEDs do not fluctuate in intensity while activated.

As explained below, our answer is it would be permissible for an optically combined parking/turn signal lamp to alternate its parking lamp and turn signal lamp LEDs while the turn signal is activated (which is the case under Alternative A), but it would not be permissible for the lamp to completely deactivate the parking lamp LED while the turn signal is activated (Alternative B).





1 We note that, while you originally requested confidential treatment of portions of your interpretation request, you withdrew that request in a subsequent communication with Daniel Koblenz of my staff, so your entire interpretation request is now publicly releasable.


In addition, as explained at the end of this letter, our interpretation about the permissibility of Alternative A applies only to lamps that use LEDs, and not to vehicles using traditional incandescent lamps.

Requirements for optically combined lamps

FMVSS No. 108 governs signal lamp performance. Under S4 of the standard, “optically combined” is defined, in relevant part, as: “a lamp having… two or more separate light sources that operate in different ways, and has its optically functional lens area wholly or partially common to two or more lamp functions.” Because your lamp meets this definition, we consider it to be an “optically combined” lamp.2 Under S6.3 of the standard, optically combined lamps are only permissible “if the requirements for each lamp, reflective device, and item of associated equipment are met.”

The specific requirements that apply to turn signal lamps are found in S7.1, and the requirements that apply to parking lamps are found in S7.8. We assume for purposes of our analysis that your lamp would meet these general requirements for both its turn signal and parking lamp functions.3

According to FMVSS No. 108, parking lamps must be activated with a vehicle’s headlamps, and they must be “steady burning” at all times.4 In past interpretations, we have understood the “steady burning” requirement for vehicle lighting to be met if the lamps is “is perceived as being steady-burning” by an observer even if, as a technical matter, the lamp is not steady burning.5


Because your parking lamp’s LEDs are not, technically, steady burning in either of the alternatives you describe, the key question is whether the parking lamp in the alternatives would appear to be “steady burning” to an observer while the turn signal is activated.

In our view, Alternative A is permissible, while Alternative B is not.  This is because only Alternative A would give an observer the perception that the parking lamp is steady burning throughout the operation of the turn signal. Under Alternative A, an observer would see the turn signal alternate between two states: amber and white.  From the observer’s standpoint, at no point is the lamp completely off; whenever the amber LED is on, the white LED is off, whenever the amber LED is off, the white LED is on. Because there is no gap in LED illumination, an observer would perceive that the white LED is on for the entire time that the turn signal is activated, and that the white light it produces is combined with the amber LED’s light while the




2 S4.

3 Please note that there are luminosity requirements for optically combined turn signal and parking lamps that are different than the luminosity requirements for turn signal lamps and parking lamps that are not optically combined. These requirements are set out in S7.1.1.12.

4 Table I-a.

5 See letter to Kiminori Hyodo (Nov. 5, 2005), available at https://isearch.nhtsa.gov/files/Koito.2followup.html.


amber LED flashes. Because the white LED would appear to be illuminated at all times, the lamp in Alternative A would meet the “steady burning” requirement for parking lamps.6 (Please note that, because FMVSS No. 108 requires that the parking lamp be steady burning, if the

lamp’s amber LED becomes inactive or is otherwise unable to meet the performance requirements for turn signals, the white LED must remain steady burning at all times, including when the turn signal is activated.)

Conversely, under Alternative B, when the amber LED is off, the lamp produces no light whatsoever. Thus, an observer would perceive that the lamp deactivates for the duration of the turn signal’s activation, which, as noted earlier would violate the “steady burning” requirement for parking lamps.


Please note that this interpretation applies only to optically combined parking and turn signal lamps that use LEDs. This is because, unlike traditional incandescent lamps, LEDs are capable of turning on and shutting off almost instantaneously. Traditional incandescent lamps take some time to power up and shut off, which means that if an optically combined incandescent lamp were to flash according to Alternative A, there would be observable gaps in illumination while the lamp is powering up. Because an observer would be able to perceive these gaps, an observer would not perceive the lamp to be steady-burning. Therefore, Alternative A would not be permissible for an incandescent lamp.

If you have any questions, please contact Daniel Koblenz of my staff at (202) 366-2992.






     Date: 2020.06.11 14:16:32




Jonathan C. Morrison Chief Counsel


Dated: 6/11/20

Ref: FMVSS No. 108

















6 It is NHTSA’s understanding that, due to the photometric differences between LEDs and incandescent light sources, if the white LED were to stay illuminated while the amber LED flashed, the combined light of the two LEDs would appear to be pink in color, rather than amber.