Observers' judgments of the effects of glare on visual acuity

Traffic collisions and pedestrian fatalities increase significantly when driving at night. There is a need for greater roadway visibility when driving at night and the use of high beam headlights can significantly improve the distance at which drivers recognize objects along the roadway. However, research suggests that drivers underuse their high beams. It is possible that drivers do not use their high beam headlights in an effort to minimize glare to oncoming vehicles. The purposes of this experiment were to extend earlier research indicating that the visually impairing effects of glare may often be exaggerated and to investigate the role of stimulus contrast and size in observers' judgments of the effects of glare. Participants were asked to judge the luminance of a glare source sufficient to impair their visual acuity of a target viewed through this glare source; these estimated glare thresholds were compared to the participant's actual glare thresholds. Participants overestimated the intensity of glare required to produce a decline in their visual performance. On average, estimates of glare threshold were 88% lower than actual glare threshold values. Participants took stimulus size into account when making their estimates of glare threshold but did not seem to consider stimulus contrast information when making these judgments. The results of the current study confirm the trend seen in earlier work indicating that drivers exaggerate the debilitating effects of glare and are not fully aware of the actual effect of glare on their visual performance.