The same principle applies to roadway lighting at night. Glare from distant fixtures is far more harmful to visibility than marginal illumination level. They will be able to see hazards within the effective area much more clearly from a much greater distance with the elimination of the direct glare. Everyone benefits from properly shielded luminaires, but special benefit comes to senior citizens and persons with slight sight impairment. Seniors, in particular, experience eyesight deterioration which makes glare difficult to accommodate.
Glare impairs their seeing ability much more than a low level of illumination.
Even though an area-lighting fixture can provide effective illumination no farther than about three times the pole height, it can cause harmful glare to a very much greater distance because the glare is a direct ray rather than being reflected off intervening objects such as the roadway, pedestrians, animals, vehicles, or hazards on the road. Their recommendations upon which most U.S. lighting standards are based - cover virtually every situation for which lighting is needed. However, even their members are just beginning to recognize the importance of direct glare elimination and some of their older publications have not yet been brought up to date in addressing this matter. Warren Offutt in an amateur astronomer and retired engineering executive residing in southern New Mexico.