Mr. Eugene Farber
S&HFC Co-Chair
SAE International
31811 Vallen Court
Beverly Hills, MI 48025

Re: Request for Interpretation of S5.5.4, FMVSS No. 108

Dear Mr. Farber:

This is in reply to your letter of February 21, 2000, asking for an interpretation of S5.5.4 of Federal Motor Vehicle Safety Standard No. 108, Lamps, Reflective Devices and Associated Equipment.

Paragraph S5.5.4 requires in pertinent part that "The stop lamps shall be activated upon application of the service brakes." You have asked two questions with regard to this requirement.

Your first question is whether it is "permissible to illuminate stoplamps when the service brakes are automatically applied (i.e., the driver does not depress the brake pedal) for purposes of whole vehicle deceleration." The SAE Standards on stop lamps that are incorporated by reference, J586 MAY84 and J1398 MAY85, define stop lamps as "Lamps . . . [which] indicate the intention of the operator of a vehicle to stop or diminish speed by braking." Given that your stated purpose of either manual or automatic service brake application is to diminish vehicle speed, the stop lamp must be illuminated. Vehicles whose service brakes are applied with a resulting diminishing of speed would be noncompliant if the stop lamps did not illuminate.

Your second question is whether it is "permissible that the stoplamps be illuminated when other mechanisms such as transmission downshifting or engine retarders are used to achieve whole vehicle decelerations in excess of the normally achievable coast-down deceleration." On

April 10, 1992, this Office informed Lance Watt of the Flxible Corporation that Standard No. 108 does not require that the stop lamps be activated when a brake retarder is activated. We appear not to have addressed transmission downshifting, but we see no difference in functional effect between it and an engine retarder. While the Watt interpretation appears inconsistent with our views in the preceding paragraph, because, in either case, the vehicle would decelerate at a higher rate than normal coastdown, the former involves brake application, while this one does not. It is possible that, upon further review, the Watt interpretation might be superceded by rulemaking to clarify the conditions under which the stop lamps must be illuminated.

We note that S5.1.3 of Standard No. 108 provides that "No additional lamp, reflective device or item of associated equipment shall be installed that impairs the effectiveness of lighting equipment required by" the standard. In this case, when a vehicle is slowed by downshifting or an engine retarder, from the perspective of the following driver, it would be equivalent to what would occur if the service brakes were applied. Therefore, it would be permissible for the stop lamps to be illuminated under this scenario, since such illumination would not create any confusion in the mind of a following driver and thus would not "impair the effectiveness" of the required stop lamps.

If you have any further questions, you may refer them to Taylor Vinson of this Office (202-366-5263).

Sincerely,
Frank Seales, Jr.
Chief Counsel
ref:108
d.5/26/00