Night blindness

Many people who score 20-20 for acuity in daylight do not test nearly so well in dim light, and may have poor ability to perceive contrasts and use peripheral vision as the light fades, or be troubled by nighttime glare.

The brighter an object is in contrast to its background, the farther it can be seen. Aspects of vision which are crucial for driving a car are contrast sensitivity (the ability to discern an object from its background), and dynamic acuity (the ability to track an object as it moves) and guidance vision (the kind of subconscious seeing that allows people to navigate while driving down a road or walking in a hall). Guidance vision works well even in total darkness, but contrast sensitivity falls precipitously as the light fades, and with it dynamic acuity.

Of 300 Canadian research subjects, one third between the ages of 15 and 25 experienced a loss of vision at night, and about 50% of these did not realize they had a problem. Low light conditions are the most common cause of all accidents and may account for about 80% of fatalities involving bicyclists or pedestrians. In the USA in 1990, nighttime automobile accidents were about 3.7 times more common per kilometre driven than daytime accidents.