Researchers affiliated with Johns Hopkins University in Baltimore looked at changes in vision, cognition and the general health status of more than 1,200 licensed drivers ages 67 to 87 in Salisbury, Md., a community with limited public transportation.

Lisa Keay said the study was unique because comprehensive tests of both vision and cognitive function were performed.

The study, published in Investigative Ophthalmology & Visual Science, found that after one year, 1.5 percent of the drivers had given up driving and another 3.4 percent had restricted their driving.

The most common predictors of stopping or decreasing driving were slow visual scanning, psychomotor speed and poor visuo-constructional skills, as well as reduced contrast sensitivity -- ability to detect detail in shades of gray -- skills necessary for drivers to be aware of and respond to other cars, road conditions and road signs.

"As a society, we would like to think that when a driver recognizes that his or her functions related to vision or cognition are declining, they make that crucial decision," Keay said in a statement. "My colleagues and I found it reassuring that in this group, that appeared to be the case."