Specialty Manufacturing Co.
P.O. Box 790
10200 Pineville Road
Pineville, NC 28134
Dear Mr. Swanger:
This responds to your request for an interpretation of the conspicuity requirements in Federal Motor Vehicle Safety Standard No. 131, School Bus Pedestrian Safety Devices. According to your letter, a State has requested that you use light-emitting diodes (LEDs) to outline the word "STOP" on the stop arm blade. That State believes such lighting would increase the sign's conspicuity in certain weather conditions. In your letter and in an October 2, 1995, telephone conversation with Mr. Paul Atelsek of my staff, you expressed your concerns that using LEDs on stop signal arms may cause confusion and asked whether they are permitted. You raised three specific issues relating to viewing angles, legibility from certain distances, and inconsistencies among various jurisdictions.
The short answer to your question is that the LEDs could comply with our standard, but only under certain conditions. As you are aware, S5.3 Conspicuity states that "The stop signal arm shall comply with either S5.3.1 or S5.3.2, or both." Either method of providing conspicuity is by itself sufficient. I will discuss how the presence of LEDs relates to each of these options and then address your specific questions below.
Section S5.3.1 sets forth the requirements of the reflectorization option, stating that "[t]he entire surface of both sides of the stop signal arm shall be reflectorized with type III retroreflectorized material . . . ." LEDs would appear on the surface of the arm but could not, as far as we know, qualify as type III retroreflectorized material. Therefore, LEDs are not permissible when compliance depends upon the reflectorization option.
Section S5.3.2, which references S6.2, sets forth requirements addressing flashing lamps. Section S6.2 specifies the lamp's color, flash rate, and on-off time. These rather specific requirements reflect the importance of consistency in any signage or labeling requirement. However, we do not see anything intrinsic about LEDs that would preclude their use in stop signal arms with flashing lamps. As long as the familiar flashing lamps are used, we do not believe that interstate confusion would result from the addition of LEDs. Note that we do not consider the use of LEDs as an "optional" method of compliance with S5.3.2, because the LEDs would not be centered on the vertical centerline at the top and bottom of the stop arm.
You expressed concerns in your letter about the narrow viewing angle of LEDs compared to incandescent lights, and about the legibility of the LEDs at a distance. Since the LEDs would be used as a supplement to a standard method of compliance (i.e., flashing lamps), a diminished viewing angle is not important. We assume manufacturer's quality control practices would prevent uneven viewing angles from LED to LED within a given stop arm. While your concerns about the legibility of the word "STOP" at a distance are important, they do not seem to relate to the presence or absence of the LEDs unless the LEDs reduce the legibility of the word. If you have data indicating that the size or spacing of the letters needs to be increased to achieve greater legibility at a distance, you may petition NHTSA to revise the standard.
I want to raise one potential safety issue, in case you receive a request to design an LED-equipped stop signal with flashing lamps. Certain arrangements of LEDs might affect compliance by impairing the effectiveness of the stop signal arm's flashing lamp. Very closely spaced red LEDs could enhance the readability of the letters in poor visibility conditions. On the other hand, red LEDs spaced every few centimeters around the outline of the 15 cm high letters could appear as a random field of lights (like a Christmas tree), distracting the observer and resulting in diminished readability. Similarly, different flash rates or on-off speeds from installed incandescent lamps might detract from readability by creating a distracting double-flash effect, as you suggest. Whether a particular LED-equipped stop signal arm complies with Standard No. 131 is a matter that can be determined only in the context of an enforcement proceeding.
I hope this information is helpful. If you have any further questions, please feel free to contact Paul Atelsek at this address or by telephone at (202) 366-2992.
Samuel J. Dubbin Chief Counsel ref:131 d:11/21/95