3920 West 23 Avenue
British Columbia V6S 1L2
Dear Mr. Rock:
This is in reply to your email of October 1, 1996, asking two questions about rear lighting on motor vehicles.
Your first question is why the agency has not required rear turn signals to be amber rather than allowing a manufacturer to choose between red and amber. Historically, red has been the color preferred by American manufacturers, while the practice in countries outside the United States has been to use amber. Intuitively it would appear that a separate amber lamp might provide a more effective signal than a red signal emitted by a combination stop/taillamp. However, Standard No. 108 compensates for a lack of separation in a rear combination lamp by requiring s the red turn signal have a higher candela than the taillamp to differentiate it, and specifies that the turn signal will override the stop lamp when it is activated. Although our field studies do show a very slight improvement in signal detectability when amber is used as a separate turn signal, the improvement is insufficient to warrant eliminating red as an acceptable color for rear turn signals, and requiring amber as the sole permissible color.
Your second question is why the agency doesn't require taillamps to be at the outer extremities of vehicles. We are aware that it is the practice of some countries to specify dimensional locations for rear lamps (e.g., within 3 inches of the edge of the vehicle), but Standard No. 108 is drafted so as to afford a manufacturer freedom to locate rear lighting as the manufacturer chooses, within the broad directive that such lamps be "as far apart as practicable." This is the requirement for the location of rear stop, turn signal, and taillamps. A manufacturer may choose to stack the lamps or to locate two of the rear lamps inboard of the third. The agency will not contest the manufacturer's determination unless it is clearly erroneous.
If you have any further questions, Taylor Vinson of this Office will be pleased to answer them (202-366-5263) or email Tvinson@NHTSA.DOT.GOV.
Acting Chief Counsel