Mandatory vision screening for Florida drivers over the age of 80 may be associated with lower death rates from traffic crashes in this age group, a new study says.

Vision screening for drivers over age 80 is a Florida law, passed in 2004, and requires all people 80 and older to pass the exam before they can renew their driver's license. For this study, researchers from the University of Alabama at Birmingham analyzed 2001-06 data on motor vehicle collision traffic deaths among all drivers in Florida. They then compared those rates to neighboring states Alabama and Georgia, which don't require vision tests for elderly drivers.

From 2001 to 2006, overall motor vehicle collision death rates in Florida increased by 6 percent, from 14.61 to 14.75 per 100,000 people per year. However, death rates among elderly drivers decreased by 17 percent, from 16.03 to 10.76 per 100,000. In Alabama and Georgia, there were no changes in death rates among older drivers.

The study was published in the November issue of the journal Archives of Ophthalmology.

While a number of possible reasons could explain the death rate decline in Florida, study author Gerald McGwin Jr. and colleagues suggested, "the most apparent reason is that the screening law removed visually impaired drivers from the road. However, in reality, the situation is significantly more complex."

The researchers noted that about 93 percent of elderly drivers were able to renew their license, which indicates that only a small percentage were denied licenses, because they failed to meet the vision standards.

In addition, it's possible the vision screening law improved elderly drivers' visual function overall, because many who failed the first test sought vision care and returned with improved vision. It's also possible that those with poor vision didn't even bother to apply for license renewal.

"Ultimately, whether the vision screening law is responsible for the observed reduction in fatality rates because of the identification of visually impaired drivers or via another, yet related, mechanism may be inconsequential from a public safety perspective," the study authors wrote. "However, the importance of driving to the well-being of older adults suggests that isolating the true mechanism responsible for the decline is, in fact, important."

Identifying this mechanism would enable states to introduce laws that accurately target high-risk older drivers while allowing low-risk older drivers to keep their licenses and mobility, the researchers said.