The safety significance of improperly aimed headlights, the mechanisms by which they develop misaim, and measures by which the generation of misaim may be avoided were studied. Surveys of headlamp conditions in states with and without vehicle inspection were made. The effect of varying degrees of misaim on the driver's ability to see roadside objects with and without opposing traffic was determined. Upward misaim of the low beams increased the level of illumination to the front targets at a greater distance, although it increased glare to opposing motorists. Downward misaim reduced glare to opposing motorists but also reduced target visibility distance. With high beams, either upward or downward misaim decreases visibility distance. Sources of headlamp misaim included: internal changes within the lamp; in-service changes in the vehicle; vehicle loading and operation; headlamp inspection and adjustment; and headlamp replacement. The most serious cause of misaim is that caused by changing the loading of the vehicle. Data are provided for predicting visibility with prototype headlamp systems. (A)
Abstract