SELECTIVE VISUAL DEGRADATION AND THE OLDER DRIVER

Drivers of all ages experience serious visual impairment in low illumination. These unavoidable visual losses seem to be of little concern to younger drivers as their night driving behavior changes little to compensate for their diminished capabilities. In contrast, older drivers are inclined to avoid night driving, and this concern seems to arise largely because they are disturbed by limitations of their vision in low light. Differences between younger and older drivers are examined here in light of the "two-visual systems hypothesis", which emphasizes that not all visual functions are equally degraded by reduced illumination. From this viewpoint, the excessive confidence of younger drivers at night has been attributed to the selective nature of visual losses in low light, when "focal" functions like acuity and contrast sensitivity deteriorate, but "ambient" functions like visual orientation and control of locomotion remain highly efficient. This account is not complete, however, until it can also explain the more cautious behavior of older drivers. The present paper moves toward a more general theory of night driving behavior through consideration of (1) age-related changes in basic visual processes which occur gradually across the adult life-span, (2) recent investigations of the role of vision as a contributing factor in specific classes of fatal road accidents; and (3) new evidence suggesting that "ambient" as well as "focal" visual functions are affected by aging. Though tentative, the "selective degradation hypothesis" promotes a realistic analysis of the role of vision in night driving.