Driving an automobile is primarily a visual task, and vision contributes as much as 90% of the information needed to drive (Alexander and Lunenfeld 1990). Night-time driving poses a special challenge, since even at night drivers need to be able to see traffic control devices, lane lines, vehicles, pedestrians, animals, and other potential hazards. Artificial lighting can illuminate the roadway, but too much light or improper lighting may result in glare, which causes visual discomfort and a diminished ability to see the environment. There are only two practical methods for night-time lighting: fixed overhead lighting and vehicle headlights. While the number of roads with fixed overhead lighting increases each year, this form of lighting is expensive and cannot be relied upon as the only means for providing night visibility. From its inception, the use of headlights on automobiles has involved a compromise between providing enough light for drivers to see the road ahead and avoiding the excessive light that produces glare. Changes in headlamp designs that affect light intensity, beam pattern, and aiming have significantly improved night vision on the highway. Technology has brought changes to headlights, interior surfaces (including mirrors), and the highway environment that directly reduce glare or indirectly reduce the effect of glare on the driver. However, every change has involved a trade-off with hidden costs. For example, lowering headlamps may reduce glare but can result in a loss of forward visibility. The framework for all subsequent discussions of glare in roadway lighting was created by Holladay (1926) in the USA and by Stiles (1928 - 29) in the UK. Holladay originated the concept of “disability glare,” glare that decreases a driver’s ability to see clearly. Stiles showed that disability glare is caused by the scattering of light in the optic media of the eye, rather than occurring in the optical nerve, as Holladay had assumed. Holladay acknowledged Stile’s explanation but noted that it did not explain all of glare’s effects. This exchange apparently began the present distinction between disability glare and discomfort glare. This report provides the reader with a working knowledge of glare and the methods used to measure and control glare. It is a rather technical report aimed at engineers and experts in lighting and traffic safety. (A)
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