Mr. Robert G. Mills
Triumph Designs Limited
Normandy Way, Hinckley
Leicestershire LE10 3BZ
Dear Mr. Mills:
This responds to your letter, dated April 8, 2011, asking whether the motorcycle rear lamp system you describe is permissible under Federal Motor Vehicle Safety Standard (FMVSS)
No. 108, which governs lamps, reflective devices, and associated equipment on vehicles. In a February 2012, meeting with agency staff you stated that the project for which you were requesting this interpretation was on hold. On October 5, 2015, you emailed Thomas Healy of my staff asking about the status of our response to your letter. I apologize for the delay in our response. As explained below, we believe that the system would be permissible under FMVSS No. 108.
By way of background, the National Highway Traffic Safety Administration (NHTSA) is authorized to issue Federal motor vehicle safety standards that set performance requirements for new motor vehicles and items of motor vehicle equipment (see 49 U.S.C. Chapter 301). NHTSA does not provide approval of motor vehicles or motor vehicle equipment, and we do not determine compliance of a vehicle or item of motor vehicle equipment outside the context of an actual enforcement proceeding. Instead, manufacturers are required to self-certify that their products conform to all applicable safety standards that are in effect on the date of manufacture. The following represents our opinion regarding the applicability of our regulations to your proposed lighting configuration based upon the facts set forth in the materials that you submitted.
In your letter and the attached diagrams, you describe the lamp system as consisting of two lamps, with one lamp located on either side of the rear vertical centerline of the motorcycle. The center axes of the lamps are separated by a distance of 315 mm (12.4 in). Each of the lamps functions as turn signal, stop lamp, and taillamp. The operational logic chart attached to your letter indicates that the system has four functional modes: 1) both lamps continuously illuminated as taillamps; 2) one lamp flashing as a turn signal while the other lamp remains continuously illuminated as a taillamp; 3) both lamps illuminated at a higher intensity as stop lamps when the brakes are applied; and 4) in a situation in which the brakes are applied at the same time as a turn is indicated, one lamp flashing as a turn signal while the other lamp remains continuously illuminated at a higher intensity as a stop lamp. You state that the effective projected luminous lens area of the two lamps in your configuration, when combined, is 50 cm2.
You ask whether it would be permissible to combine the two lamps on either side of the vertical centerline of the motorcycle in order to meet the effective projected luminous lens area requirements of FMVSS No. 108. You further inquire whether the required minimum 4 inch edge to edge separation of red turn signal lamps from the taillamp or stop lamp for motorcycles equipped with a single stop and taillamp applies to your lighting configuration.
You pose an additional question about the operating condition of the lamps when both the turn signal and stop lamps are activated. In your letter, you state that when one of the turn signal lamps is activated during braking, only the lamp on the opposite side of motorcycle from the turn signal that is flashing will be activated as a stop lamp. Since only one of the two lamps used to meet the effective projected luminous lens area requirement for the stop lamp is activated in this situation, the effective projected luminous lens area falls below that required for a motorcycle equipped with a single stop lamp. You inquire whether this situation would be permissible under FMVSS No. 108.
We agree that you may combine the lamps on either side of the vertical centerline of the motorcycle for the purpose of meeting the effective projected luminous lens area requirements for a motorcycle equipped with a single stop lamp in FMVSS No. 108. FMVSS No. 108 requires that the stop lamps and the rear turn signal lamps must meet the requirements of Table IV-a. FMVSS No. 108 permits the use of multiple compartment lamps or multiple lamps to meet the photometric requirements for stop lamps. The compartments or lamps in such systems are tested together as a unit as long as all the compartments or lamps are within a certain distance of each other. For a two-lamp system, the center axis of the lamps must be within 560 mm of each other to be tested as a unit. The distance between the two lamps in your configuration is less than 560 mm, thus the lamps could be considered a combination lamp for the purpose of meeting the effective projected luminous lens area requirement for the stop lamp.
In previous interpretation letters addressing rear motorcycle lamp configurations, we have applied the distance requirements in FMVSS No. 108 and referenced SAE standards to conclude that lamp systems consisting of lamps on either side of the rear vertical centerline of a motorcycle can be considered single lamps for the purposes of meeting the stop lamp photometric requirements (See enclosed November 20, 1998 letter to Tadashi Suzuki).
You correctly note in your letter that Table I-c of FMVSS No. 108 requires turn signal lamps on a motorcycle to be separated from the tail lamp or stop lamp by 4 inches when a single tail or stop lamp is mounted on the vertical centerline of the motorcycle and the turn signal lamps are red. We do not believe that this requirement is applicable to the configuration described in your letter because the configuration you describe consists of two lamps mounted either side of the vertical centerline instead of a single stop or tail lamp mounted on the vertical centerline of the motorcycle.
We believe that the situation in which the turn signal and stop lamp of your proposed lighting system are both activated would be permissible under FMVSS No. 108. The Table I-c of FMVSS No. 108 states that when a stop lamp is optically combined with a turn signal lamp, the circuit shall be such that the stop signal cannot be turned on if the turn signal is flashing. According to the definition of “optically combined” in FMVSS No. 108, optical combination results when 1) a lamp has two or more separate light sources, or a single light source that operates in different ways (e.g., a two-filament bulb), and 2) the lamp’s optically functional lens area is wholly or partially common to two or more lamp functions.
In your proposed system, both lamps together constitute the required single stop lamp, and each individual lamp also acts as a turn signal. Under the definition stated above, the single stop lamp (consisting of both lamps) is considered to be optically combined with both turn signals. Such an interpretation could mean that a situation when one lamp flashes as a turn signal while the other lamp remains continuously illuminated as a stop signal would not be permissible. Neither of the lamps could be illuminated as a stop signal if one of the turn signals is flashing. In other words, the vehicle would not display any signal indicating that the brakes were being applied in such a situation. If the taillamps could not indicate braking, we would consider this to constitute a safety risk and to be impermissible under FMVSS No. 108.
In the unique situation presented by the lamp system you describe, when one lamp flashes as a turn signal while the other lamp remains continuously (and more strongly) illuminated as a stop signal, the lighted section that is flashing as a turn signal does cease to operate as a stop signal. The lamp system as a whole, however, would continue to signal when the brakes are applied because the other lighted section continues to operate as a stop signal. We conclude that the situation when one lamp flashes as a turn signal while the other lamp remains continuously (and more strongly) illuminated as a stop signal is permissible under FMVSS No. 108. We caution that this interpretation is limited to the unique motorcycle rear lamp system described in your letter.
If you have further questions, you may refer them to Thomas Healy of this Office (202-366-2992).
Paul A. Hemmersbaugh
Acting Chief Counsel
Ref: Standard No. 108
 See 49 C.F.R. 571.108 S18.104.22.168.