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The purpose of this work is to determine as a function of velocity the minimal roadway luminance that is required to be judged as being bright enough for a driver to perform a nighttime driving task with an adequate feeling of safety. In this context, it shall also be evaluated which areas of the vehicle forefield are most crucial for the driver’s general brightness perception. A field study with 23 subjects and dimmable LED headlights was conducted, in which the subjects were given the task to assess their perceived brightness for different luminance levels caused by the headlights’ low-beam distribution in the vehicle’s forefield on a 5-step rating scale. The experiments were repeated for three different driving velocities of 0kmh⁻¹ (static case), 30kmh⁻¹, and 60kmh⁻¹, respectively. Results for the static case indicate that, for the roadway to be perceived as bright enough by 50% of the subjects, an average roadway luminance of 0.88 cdm⁻² is required in an area up to 32m in front of the vehicle. Furthermore, a significant effect of driving speed is observed. For example, at 60kmh⁻¹, the luminance must be increased to 1.54 cdm⁻² to be still perceived as sufficiently bright by 50% of the subjects.

Keywords: nighttime driving; roadway luminance; increased feeling of safety; brightness perception; adaptive headlights; automotive lighting