Mr. Jack W. DeYoung
Tumbleweed Trucks, Inc.
318 Kwanzan Drive
Lynden, WA 98264
Dear Mr. DeYoung:
This is in reply to your letter of October 18, 2002, with respect to the flash rate of hazard warning signal system flashers. You have invented a flasher "which is designed to produce a hazard signal consisting of repeating cycles of a number of short flashes followed by a longer pause." Your question, in essence, is whether Federal Motor Vehicle Safety Standard No. 108 requires "a uniform flash rate" for hazard warning signal system flashers or permits a varying one. It is your opinion that the standard does not require a "uniform flash rate."
Standard No. 108 requires that hazard warning signal flashers be designed to conform to SAE Recommended Practice J945, "Vehicular Hazard Warning Signal Flasher," February 1966. Paragraph 3 of J945 and its accompanying Figure 1 specify requirements for "Flash rate and percent Current On" Time" measurements." The flash rate must be 60 to 120 flashes per minute for "normally open" (i.e., variable load) flashers, and 90 to 120 flashes per minute for "normally closed" (i.e., fixed load) flashers. SAE J945 also requires that "flashing rate and percent current on time . . . be measured after the flashers have been operating for a minimum of five consecutive cycles and shall be an average of at least three consecutive cycles."
We have examined the diagram in your letter relating to your flasher invention.It shows that this design would not comply with Standard No. 108. We calculate that its flash rate would be 136.8 flashes per minute, exceeding the maximum permissible 120 flashes per minute. Specifically, the diagram shows two consecutive cycles of 160ms "on" followed by 200ms "off." These cycles are followed by a third cycle of 160ms "on followed by 620ms "off." The first two cycles equate to 166.7 flashes per minute, while the third cycle equates to 76.9 flashes per minute. The average of the three cycles is 136.8 flashes a minute.
While you might be able to modify your invention to reflect a complying flash rate for three specific cycles, we interpret the standard as requiring compliance over any three cycles chosen at random. The drastic change that would appear required for you to comply at three specific cycles would sharply reduce (and possibly eliminate entirely) the ability to comply at another three cycles chosen at random. Further such a change in your design would, we believe, affect the ability of the flasher to meet the percent on-time requirements. The need to fulfill both flash rate and percent on-time requirements may explain why industry has always chosen a uniform rate for flashers. Thus, while you are correct that SAE J945 does not specify that all cycles of flash be of the same duration, it is likely that the only way to meet the specification is to provide a "uniform flash rate."
I hope that this answers your questions.