Interpretation ID: 6972
Director of Engineering
The Flxible Corporation
970 Pittsburgh Drive
Delaware, Ohio 43105-2859
Dear Mr. Watt:
This responds to your letter of February 11, l992, asking for an interpretation of Motor Vehicle Safety Standard No. 108 as it relates to several scenarios regarding the wiring and use of optional brake retarder transmissions on city transit buses manufactured by your company.
In your current design, the retarder is designed so that it is electrically operated during the initial travel of the service brake pedal. As the service brake pedal is further depressed, the service brakes are activated, and this in turn illuminates the stop lamps. You have enclosed a copy of my letter of September 20. 1990, confirming that this design conforms to Standard No. 108, specifically S5.5.4 which states that "The stop lamps on each vehicle shall be activated upon application of the service brakes."
Since that time, several additional scenarios have presented themselves. First, some customers have requested that the transmission retarder be activated when the accelerator pedal is released, rather than when the brake pedal is applied. In this configuration, the stop lamps would not be illuminated, "and therefore, following vehicles may be unaware of this sudden reduction in vehicle speed", unless the service brakes were also applied. However, some customers wishing this option would like to have the stop lamps illuminated by the retarder, that is to say, when the accelerator is released. Second, some customers have also requested a retarder cut-off switch in order to disable the retarder during icy or slippery road conditions. In such a case, the stop lamps would also be activated at the time of accelerator release "with minimal if any change in vehicle forward speed, and again, potentially with no intent on the part of the driver to use the service brakes." To date, your company has resisted these requests, but these customers, without a specific NHTSA interpretation on the point, threaten to declare your company a nonresponsive bidder on transit bus procurements. You have asked whether a noncompliance with section S5.5.4 would result "if the stop lamps were activated without depressing the brake pedal as requested by our customers."
The purpose of the retarder feature is to provide supplemental braking to city transit buses. This braking results in the deceleration of the vehicle. A stop lamp is defined by SAE Standard J1398 MAY85 Stop Lamps for Use on Motor Vehicles 2032 mm or More in Overall Width as one that indicates "the intention of the operator of a vehicle to stop or diminish speed by braking." Whenever the brake retarder is activated with the intent of diminishing speed by braking, Standard No. 108 does not require that the stop lamps be activated. The only mandate of the standard (S5.5.4) is that when the service brakes are applied, the stop lamps must be illuminated.
Nor does Standard No. 108 prohibit illumination of the stop lamps by release of the accelerator pedal followed by activation of the retarder. This is because the intention of the driver is to diminish speed by the braking action of the retarder. We distinguish this situation from the one in an interpretation provided Larry Snowhite, Esq. on January 25, 1990, in which a device activated the stop lamps whenever the accelerator pedal was released, regardless of the intent of the driver. Activation of the stop lamps initiated by release of the accelerator pedal is permissible only when the intent of the driver is to reduce the speed of the vehicle by an immediate subsequent act of braking, whether that is achieved through his use of the service brake system, use of retarders, or a combination of the two.
However, a configuration where the stop lamps operate in the absence of service brake application or activation of a retarder system (as appears to occur when a retarder cut off switch has been activated) would be subject to S5.1.3 of the standard. This prohibits the installation of motor vehicle equipment that impairs the effectiveness of the lighting equipment required by Standard No. 108. In this instance, the retarder cut off feature would permit the stop lamps to send the false signal that the operator intended to stop or reduce vehicle speed when, in fact, there was no intent to do so.
I hope that this answers your question.
Sincerely,
Paul Jackson Rice Chief Counsel
ref:108 d:4/10/92