The vision standard presently used to evaluate drivers' vision is snellen visual acuity. Visual acuity, however, has not been found to relate well to everyday visual performance such as driving. Because acuity tests the optics of the eye--the ability of the eye to focus an image--it is a measure of quantity, not quality, of vision. Furthermore, the black letters on the white background impose a high-contrast test condition that cannot be related to common low-contrastsituations found in the real world like dusk, fog, or rain. However, contrast sensitivity, a more comprehensive method of evaluating vision, has recently been shown to be related to visual task performance in studies involving highway sign discriminability and pilot target detection. Contrast sensitivity has also been found to more accurately correlate with functional vision loss due to a disease such ascataracts. From health and safety aspects, detecting functional vision losses is extremely important. Cataract patients, for example, may have severely impaired functional vision but still measure 20/20 or 20/40 on the standard visual acuity test. Although acuity may notshow a significant loss of vision, contrast sensitivity can detect many such losses and alert both the individual and the tester to theproblem. In sum, the serious health and safety issues presented by the inability of acuity to measure losses of functional vision and visual quality should be addressed, especially with regard to driver visual standards. A reasonable replacement for acuity appears to be contrast sensitivity, which relates to visual performance and can detect visual deficits. This paper appeared in transportation researchrecord no. 1149, Visibility for highway guidance and hazard detection. For covering abstract see IRRD no 817811.
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