Interpretation ID: Mills.1
Robert G. Mills, Supervisor, Homologation
Triumph Designs Limited
Normandy Way, Hinckley
Leicestershire LE10 3BZ
United Kingdom
Dear Mr. Mills:
This responds to your March 22, 2005, letter in which you requested clarification regarding the proper method for measuring the required edge-to-edge separation distance between a motorcycles front turn signal lamps and headlamp under Federal Motor Vehicle Safety Standard (FMVSS) No. 108, Lamps, Reflective Devices, and Associated Equipment. Specifically, your letter asked whether the minimum edge-to-edge separation distance is measured: (1) "In a 2-dimensional plane, viewed directly from the front of the motorcycle, that would only take account of the visible, vertical edge-to-edge distance between the two lamps" or (2) "In a tangential plane that would take account of the upward/downward or forward/rearward separation distance". As discussed below, our interpretation differs from the two you suggest. We interpret Standard No. 108s requirement for the separation distance between a motorcycles front turn signal lamps and headlamp as being the shortest distance between the edges of each lamps effective light-emitting surface as projected onto a two-dimensional vertical plane perpendicular to the longitudinal axis of the motorcycle (i.e. , the view that would be perceived by oncoming drivers). This projection is defined within FMVSS No. 108 as the "effective projected luminous lens area".
By way of background, Table IV, Location of Required Equipment, of FMVSS No. 108 requires motorcycle front turn signal lamps to have a minimum edge-to-edge separation distance of four inches between these lamps and the headlamp. Our interpretation that the distance is measured with a two-dimensional frame of reference is consistent with both the purpose of the standards separation requirement and at least one prior interpretation.
Standard No. 108 specifies a minimum separation distance between headlamps and turn signal lamps to minimize the possibility that an observer will not see the turn signal. A motorist approaching the motorcycle in oncoming traffic will perceive the required four-inch edge-to-edge separation distance in a two-dimensional plane. If measurement along a tangential plane were substituted, as in the second method identified in your letter, the perceived separation distance in this scenario may be reduced to less than four inches, as seen by the driver, and a situation could arise in which the conspicuity of the turn signal is masked by the headlamp beam. If the oncoming driver does not perceive the important information provided by the turn signal, traffic safety could be compromised.
The minimum edge-to-edge separation distance is the shortest distance between the edges of the effective projected luminous lens areas of the two lamps. "Effective projected luminous lens area" is defined in S3 of FMVSS No. 108 as "the area of the orthogonal projection of the effective light-emitting surface of a lamp on a plane perpendicular to a defined direction relative to the axis of reference". In this case, the defined direction is the longitudinal axis of the motorcycle.
This same reasoning and result were applied in our interpretation letter of April 23, 1986 to a party whose identity was kept confidential (see enclosure). That letter dealt with the plane of reference for measuring the separation distance between a motorcycles rear turn signals and stop/tail lamp and the view provided to traffic approaching from the rear. However, the principles and reasoning are otherwise identical to the forward-facing situation.
We would also point out an important additional requirement for motorcycle turn signal lamp placement contained in Table IV. That requirement is that the turn signal lamps must not be closer than 16 inches, as measured from their centers.
If you have further questions, please feel free to contact Eric Stas of my staff at this address or by telephone at (202) 366-2992.
Sincerely,
Jacqueline Glassman
Chief Counsel
Enclosure
ref:108
d/5/3/05