When's the right time for an older adult to stop driving? Researchers at the University of Western Australia sought to shed light on this question by investigating at what point declining vision increases the risk of a car accident. What they found is that specific types of defects in an older person's field of vision are associated with an increased risk of car crashes. As the number of American seniors who have driver's licenses has skyrocketed to nearly 48 million over the last 20 years, it's a research question with real-world implications. The new study will be presented this week at AAO 2023, the 127th annual meeting of the American Academy of Ophthalmology.

Each state in the US has different driving laws, but states generally require drivers to have 20/40 vision or better in at least one eye to drive unrestricted. But some studies suggest that other measures of vision, such as visual field, are as important as how well a person sees on the "Big E Chart."

The eye chart tests for visual acuity, or our ability to see distant objects clearly. Visual field is how wide we can see when staring straight ahead. A wide field of vision allows drivers to see enough details on either side of a car while still paying attention to what is in front of them.

This is the first population-based investigation to compare a largescale ophthalmic database of visual fields for older adults with police-reported crash, licensing and hospital morbidity data. Researchers evaluated 29 years of data from more than 31,000 drivers in Western Australia aged 50 and older. Over 4,000 older drivers, accounting for 14 percent of older drivers in Western Australia, were involved in at least one car crash. More than half of those in a crash were experiencing some extent of visual field loss.

Results suggest that the area of vision affected, and severity of it contributed significantly to crash risk:

• Visual field loss of any sort in both eyes increased the odds of a car crash by 84 percent.
• Moderate visual field loss in one eye increased car crash risk only if it occurred in the left upper or lower quadrants.
• Severe vision loss in any quandrant increased the chances of an accident.
• Central vision loss in either eye was not associated with an increase in car crash incident.

"Many people think that only good visual acuity or clarity of vision is necessary for safe driving," said lead researcher Siobhan Manners, MD. "We hope these results will help educate the public about the importance of having an adequate field of vision to be able to continue driving safely. We also hope to better inform clinicians, licensing authorities, and people with visual field defects of the thresholds for visual field loss that still allows for safe driving," Dr. Manners said.