Fig. 2. Visual acuity (in minutes of arc) as a function of target contrast at different luminance levels and for subjects of different age. A luminance level of 34.3 candelas per square meter (cd/m2) represents a minimum for adequate daytime vision. Typical highway luminances at night range from 0.343 to 3.43 cd/m2. A visual acuity of 1 minute of arc is equivalent to 20/20 Snellen. (Richards OW: Vision at levels of night road illumination: XII. Change of acuity and contrast sensitivity with age. Am J Optom Arch Am Acad Optom 43:313, 1966. Modified from Davis JK: Prescribing for visibility. Probl Optom 2:131, 1990.)