September 2010




Driving in the dark

by Matt Young EyeWorld Contributing Editor


New research leads to some intriguing conclusions about the impact of cataracts and other refractive problems on nighttime driving. "Modest amounts of visual impairment have the ability to reduce components of nighttime driving ability, including recognition and speed, and the effects are greater for simulated cataracts than refractive blur, despite the fact that the VA [visual acuity] levels were matched between conditions," wrote lead study author Joanne M. Wood, Ph.D., Queensland University of Technology, School of Optometry, Brisbane, Australia.

These results also impact the way vision should be tested prior to driving approval, Dr. Wood reported online in April in Optometry & Vision Science. "Although VA remains the most commonly administered measure of vision for driving across the world, its ability to predict both day and nighttime driving performance is not as strong as that of measures of CS [contrast sensitivity]," Dr. Wood reported. "However, it is also important to recognize that day and nighttime driving performance potentially involves other aspects of visual performance, including visual attention, detection of motion, peripheral vision and patterns of optical flow; it would be useful to incorporate these measures into future studies of vision and driving performance."

Dr. Wood analyzed closed road nighttime driving performance for 20 visually normal participants under normal vision conditions, simulated cataracts, and refractive blur. These visual conditions were simulated with modified goggles for each participant. Results contribute to the body of literature that finds concerning links between visual disease and poor driving performance.

Intriguing conclusions

Dr. Wood's study generated plenty of interesting data, but the conclusions are far more instructive of what interferes with driving performance. For one, the simulated cataract condition almost halved the mean number of signs recognized by the participantsdespite the fact that the majority of signs were retroreflective. In addition, contrast is much more important than object size when it comes to visibility while driving at night. Slow driving, meanwhile, simply can't compensate for the amount of impairment visual problems cause. "All participants drove more slowly with simulated visual impairment, with participants taking 36% longer for the cataract condition but only 7% longer for the blurred vision condition relative to normal," Dr. Wood reported. "Importantly, despite the fact that all participants drove more slowly for the visual impairment conditions, it was not sufficient to compensate for the decrease in their recognition abilities, as evidenced by the reduction in ability to detect the road signs, road hazards, and pedestrians."

The study also debunked some myths about driving performance while visually impaired. "Interes-tingly, lane-keeping ability was not significantly affected when driving with simulated visual impairment, which is in accord with our previous findings on a closed road circuit for daytime driving conditions," Dr. Wood reported.

"Similarly, nighttime driving simulator studies have shown that lane-keeping ability is relatively unaffected by refractive blur, even when the amounts of blur were extreme (up to amounts of 8 to 10 D of blur)."

When Dr. Wood compared her study to others, she found that even alcohol consumption may not be as damaging as visual impairment at times. "When adjusted for distance driven, the fatality rate at night is two to four times higher than that for daytime, and the effects are even more pronounced for fatal crashes involving pedestrians, where the nighttime pedestrian fatality rates are up to seven times higher than those in the daytime," Dr. Wood reported. "Analyses of crash databases indicate that reduced lighting and poor visibility are associated with these relatively high fatal crash rates, rather than other factors that vary between day and nighttime, such as driver fatigue and alcohol consumption."

Anastasia Pappa, M.D., Athens, Greece, said cataract is indeed a problem for some drivers, but they continue to drive. "Some can have early cataract and still be able to drive," Dr. Pappa said. But even patients with early cataract can experience halos and glare. Those with posterior subcapsular cataract in particular would experience more significant glare and halos, she said. Night blindness is another problem for some drivers, who organize their day to avoid driving in darker hours. "They come home by sunset," Dr. Pappa said.

Editors' note: Dr. Wood has no financial interests related to this study. Dr. Pappa has no financial interests related to her comments.

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