NIGHT DRIVING STUDIES

The ability to see at night requires visual skills that are not tested adequately with standard visual acuity tests. Many people who have 20/20 acuity on an E-chart test do not test nearly as well in dim light. More importantly, people who have excellent Snellen acuity scores may still experience difficulty in perceiving contrast, have poor peripheral vision as the light fades, or have trouble dealing with night-time glare.

Contrast Sensitivity Changes With Light Levels This chart illustrates contrast sensitivity functions for two different levels of mean luminance. The upper curves were measured with gratings having a mean luminance of 500 cd/m2. The lower curves were measured with gratings having a mean luminance of 0.05 cd/ m2 squared. (In references, see: Campbell FW, et al. Application of Fourier analysis to the visibility of gratings.)

 

According to National Safety Council statistics, there are 5.4 deaths per million vehicle miles driven at night, whereas there are 1.6 deaths per million vehicle miles driven during daytime. The National Traffic Highway Safety Commission discovered in 1990 that the nighttime auto accident rate was 3.7 times higher than the daytime rate. Some contributing factors are:

• Difficulty seeing contrast in poor light.
• Decreased peripheral vision when contrast sensitivity is diminished.
• Rapid recognition ability deteriorates under poor contrast.
• Visual attentions falls off at night.
• Many younger myopic drivers (20-40 years old) experience decreased nighttime vision.

(From: Recognizing poor night vision as a major problem. Herve M. Byron, MD. Ocular Surgery News, March 1, 1993.)

Driver’s License Testing & Renewal A 1990 study of Pennsylvania driver renewals found that the use of contrast sensitivity testing during vision screenings revealed instances of potentially blinding diseases in older drivers. The study also found that using contrast sensitivity testing in addition to standard visual acuity and visual field criteria testing resulted in a decrease in driving accidents and violations.

The study was conducted on a random sample of 12,710 license renewals. 73 percent of the drivers who failed the visual screening went for an eye exam and completed a follow-up questionnaire. Of those in the follow-up group, 54 percent indicated that they were unaware of their vision problems before the screening. Some of the common problems found during the contrast sensitivity screening included out-of-date corrective lenses, cataracts, glaucoma and macular degeneration. The study concluded that standard visions tests should be supplemented with contrast sensitivity testing.

 

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