Nighttime crash rates are disproportionately higher than daytime crash rates. Even though only 25% of driving occurs at night, approximately 50% of fatal crashes occur at night. Several factors contribute to the higher nighttime fatality rate including degraded visibility under dark conditions.  Human factors experts evaluate nighttime visibility and conspicuity using several methods such as field investigations and calculations of visibility level. 

When investigating nighttime collisions, it is critical to determine the distance at which a hazard (e.g., pedestrian, animal, road debris, disabled vehicle, etc.) should have been detected by the striking driver. Establishing the detection distance is critical because a driver cannot reasonably be expected to take evasive action to avoid a collision until they can, at a minimum, detect the presence of the hazard on the roadway.

Human Factors experts (often referred to as visibility experts or conspicuity experts) have specialized expertise in performing these evaluations, including conducting field investigations of nighttime visibility and calculating visibility level (VL).