A Florida law requiring all seniors over 80 to pass a vision test before renewing their driver’s license may have lead to a reduction in traffic-related fatalities among that age group. New study results show how.

Researchers compared the rate of traffic accidents and related deaths in Florida to neighboring states Alabama and Georgia, where no vision screening laws exist. They found that, between 2001 and 2006, overall traffic deaths among Florida drivers over 80 fell by 17%, while the death rate in Alabama and Georgia stayed the same. The vision screening law was passed in 2004.

Researchers at the University of Alabama at Birmingham attribute the lower death rate to a number of complicated, inter-weaving factors. Not only does the law keep those with bad eyesight off the roads, it may have encouraged some who initially failed the test to seek the help of ophthalmologists to improve their vision. Some with very bad eyesight simply might have opted not to renew their licenses at all, researchers speculate. Their report is published in the November issue of the Archives of Ophthalmology.