Interpretation ID: nht94-7.14

DATE: March 29, 1994

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: Harry C. Gough, P.E. -- Automotive Engineering Professional Specialist, State of Connecticut, Department of Motor Vehicles

TITLE: None

ATTACHMT: Attached to letter dated 12/2/93 from Harry C. Gough to NHTSA Chief Counsel (OCC 9398)

TEXT:

This is in reply to your letter of December 2, 1993, with respect to the term "alternately flashing" as it applies under Safety Standard No. 108 to school bus lamps. You ask for our opinion because a manufacturer of strobe lighting has supplied documentation indicating that the system complies with Standard No. 108.

According to your letter, in this system, the lamp on one side of the school bus (front and rear) "flashes on and off four times in a 255 millisecond period and then stays off for 745 milliseconds, then the lamp on the opposite side of the bus repeats the aforementioned pattern." You inquire as to whether "alternately flashing" refers to this pattern, "or do the four distinct on/off cycles on each side of the school bus defeat the intent of the term alternating."

As you know, paragraph S5.1.4 of Standard No. 108 incorporates by reference SAE Standard J887, School Bus Red Signal Lamps, July 1964, which requires that school bus warning lamp systems "flash alternately." We believe that the light emanating from a strobe lamp that flashes four times in 0.255 second will be perceived as a single flash of varying intensity and not as four separate flashes, and that when this is followed by an identical pattern on the other side of the bus, the system is one that is alternately flashing within the meaning of Standard No. 108.

Further, under this interpretation, the flash rate meets SAE J887's specification of 60-120 flashes a minute. Unlike other SAE materials incorporated by reference relating to signal lamps (e.g., J1133 School Bus Stop Arms in Standard No. 131 School Bus Pedestrian Safety Devices and J590b Automotive Turn Signal Flashers in Standard No. 108), J887 contains no "percent current 'on' time" requirements.

I hope that this answers your question.