FMVSS-108 says that some lights must be mounted “as high as practicable.” What is practicable?

NHTSA has stated that the vehicle manufacturer has the responsibility to choose the best mounting location “in light of the particular design/configuration of the vehicle involved” and they will not contest it unless it is “clearly erroneous.” Recently, Canada has started to strictly enforce the word practicable as meaning “capable of being done.” In the case of side markers, this means not more than six inches from the edge of the vehicle.