Accuity, sensitivity, speed of recognition and depth perception are all affected as light levels change. Statistics show that a disproportionately large number of U. S. traffic fatalities and injury accidents occur at night. For example, the ability to see a pedestrian at night is affected by multiple factors, including headlight profile, clothing material and color, street lighting, and sky conditions. Similar parameter sets can be defined for traffic signs and other traffic control devices.
Daytime visibility is an equally important area of investigation.
The effect of sun glare, for example, is to reduce the contrast of visual targets, thus washing out lines of sight on highways and at railroad crossings.