Nighttime Visibility in Varying Moonlight Conditions

When the visibility of an object or person in the roadway from a driver’s perspective is an issue, the potential effect of moonlight is sometimes questioned. To assess this potential effect, methods typically used to quantify visibility were performed during conditions with no moon and with a full moon. In the full moon condition, measurements were collected from initial moon rise until the moon reached peak azimuth. Baseline ambient light measurements of illumination at the test surface were measured in both no moon and full moon scenarios. Additionally, a vehicle with activated low beam headlamps was positioned in the testing area and the change in illumination at two locations forward of the vehicle was recorded at thirty-minute intervals as the moon rose to the highest position in the sky. Also, two separate luminance readings were recorded during the test intervals, one location 75 feet in front and to the left of the vehicle, and another 150 feet forward of the vehicle. These luminance readings yielding the change in reflected light attributable to the moon. In addition to the quantitative measurement of light contributed by the moon, documentation to the change in visibility of objects and pedestrians located on the roadway were documented through photographs. Calibrated nighttime photographs were taken from the driver’s perspective inside the vehicle with low beam headlamps activated. The photographs were analyzed after testing to determine how the light intensity of the pixels in the photographs changed at each thirty-minute interval due to the additional light contribution from the moon. The results of this testing indicate that the quantifiable change in visibility distance attributable to added moonlight was negligible, and in real-world driving situations, the effect of additional illumination from a full moon would be unlikely to affect the detection of an object or pedestrian in or near the travel lane of the roadway.