BIRMINGHAM, AL --- A law requiring vision screening for Florida drivers 80 and older was associated with lower car-crash fatality rates, but the reasons may not be vision-related, according to a new study.

Death rates from car collisions decreased 17 percent among drivers 80 and older, Gerald McGwin Jr., Ph.D., of the University of Alabama, and colleagues reported in the November issue of Archives of Ophthalmology.

Older drivers represent the fastest growing segment of the driving population. The expansion of this group has raised concerns about road safety, in light of older drivers' increased rate of motor vehicle collisions per mile driven, MedPage Today reported. Research has suggested that this increase may be partly attributed to medical, functional and cognitive impairments, but there is little evidence for an association between visual acuity and motor vehicle collisions.

Nevertheless, in 2004 Florida passed a law requiring all drivers 80 and older to pass a vision test before renewing their license. The minimum requirements for passing the test are as follows: If visual acuity in the weakest eye is better than 20/200, the applicant must have a visual acuity of at least 20/70 in the other eye or with both eyes together. If the visual acuity in one eye is 20/200 or worse, the applicant must have a visual acuity of at least 20/40 in the other eye and with both eyes together.

To determine the rates of car-crash deaths before and after the law went into effect, the researchers used data from the National Highway Traffic Safety Administration's census of all car crashes that occur on U.S. public roads resulting in the death of a person, not just the driver, MedPage Today reported. To account for any population changes that might explain changes in fatalities over time, the researchers used data from the U.S. census bureau. They also compared Florida car-crash death rates with those in Alabama and Georgia, states that did not change their legal requirements during the same time period.

From 2001 through 2006, the researchers found, there was a nonsignificant increase in overall car-crash fatality rates in Florida. But fatality rates among drivers 80 and older demonstrated a significant downward linear trend.

When comparing pre-law (2001-2003) and post-law (2004-2006) periods, adjusted for age, race and sex, the fatality rate among all drivers increased by 6 percent -- from 14.61 per 100,000 persons per year to 14.75 per 100,000 persons. On the other hand, fatalities among drivers 80 and older decreased significantly by 17 percent from 16.03 persons per 100,000 per year to 10.76 persons per 100,000.

Death rates among older drivers did not change in Alabama or Georgia during the same period.

These results suggest that states with mandated visual acuity tests may have lowered car-crash fatality rates among older individuals. If this is so, the investigators said, an important question is: How did the change in licensing requirement affect fatality rates?

Perhaps the most obvious explanation, they said, is that the law removed visually impaired drivers from the road. However, 93.3 percent of those who sought license renewal were successful. Another possibility is that the test improved visual function because 77 percent of the 12 percent of older drivers who failed the test sought vision care and were then able to pass the test.

Finally, perhaps those who considered themselves to have poor vision chose not to seek renewal, believing they could not pass the vision test. It is also possible that the observed decline in fatalities may result from a variety of other factors, none having anything to do with removing visually impaired drivers from the road, or improving the vision of those who still drive. For example, the researchers said, in-person renewal may bring problem drivers to the attention of licensing authorities, serving as a screening mechanism independent of vision testing.

Study limitations included the fact that Florida's four-year licensure renewal cycle did not include all individuals who turned 80 during this period.

It is also possible that the decline in fatalities for these older Florida drivers, which appears to have begun as early as 2001 and continued through 2004, may reflect a secular trend rather than the vision-screening law. On the other hand, researchers said there is no evidence that improved seat belt use or other safety improvements in this time frame would support such an explanation. Furthermore, no changes in fatalities were observed in Alabama or Georgia.

Future research is needed to find the true mechanism responsible for the decline in accidents. It is important to ensure that such laws do not remove visually impaired, yet potentially safe, older drivers from the road when in fact the responsible mechanism may not be vision-related, the researchers said.

To review the study, click here.