"In the UK and parts of Europe, in order to have a license, you need to be tested for central blind spots, but in the U.S. you don't have to," lead author Matthew Bronstad told Reuters Health.
In his simulator test, people with blind spots often could not respond in time to a hazardous situation, said Bronstad, an instructor of ophthalmology at Harvard Medical School in Boston.
Blind spots at the center of the field of vision, or "central field loss," are usually caused by macular degeneration, damage to the retina that occurs with age. About 1.75 million people in the U.S., most over 65 years old, have this kind of vision loss, according to a 2004 study.
For the current test, 11 people with blind spots of various sizes to the left or right of center, where pedestrians and road hazards are likely to appear, drove in a simulator for 90 minutes at the Veterans Affairs Medical Center in Boston. For comparison, another 11 people without vision loss also took the driving test.
As the drivers travelled at 30 miles per hour (mph) on a city course and 60 mph on a country course, pedestrians appeared about once per minute on each side of the road. Drivers honked the horn to indicate they had seen the pedestrian.
People with blind spots were slower to honk the horn compared to people with normal vision, and were slowest when the pedestrian appeared in their blind spot than elsewhere, according to results published in JAMA Ophthalmology.
Reaction times of people with blind spots ranged from one second - the average time for a normal vision person - to five seconds. In a few cases, the impaired drivers failed to spot the pedestrian at all.
Drivers with blind spots, who could legally drive in most U.S. states but not in the UK or Canada, reacted too late to avoid a collision 29 percent of the time, compared to three percent for those with normal vision.
In the U.S., vision requirements for driving are primarily based on acuity, measured by reading letters from a wall chart. Usually drivers have to score 20/40 or better to be granted a license. Testing does not take into account specific visual impairments.
That doesn't necessarily mean the U.S. needs more regulations - it means doctors should be able to tell their patients where their particular blind spots are and to be especially careful of obstacles, said Bronstad.
"You can have a blind spot in both eyes and still have the acuity to drive," he said. "If you just have a tiny field loss, you're not going to have the same result as someone with 20 percent reduced field of vision."
People with central field vision loss often do not know they have a specific blind spot, or how large it is, before being tested by an optometrist.
"People with age-related macular degeneration tend to limit their own driving," said Bronstad. Eight of the 11 people with blind spots in his study had stopped driving on a daily basis already.
"They're less likely to want to drive on unfamiliar roads, where it's steep, in bad weather, at night - any time it gets a little bit more stressful."
"In general older drivers' physical frailty may decrease the reaction time," Ediriweera Desapriya, who studies the safety of older drivers at the University of British Columbia in Vancouver, told Reuters Health in an email.
Other factors in aging vision may also play a role in driving ability, including perception of color and brightness, slower light and dark adaptation and slower visual processing in general.
There are ways to combat these problems with technology, such as talking GPS systems and small lenses mounted on driving glasses, called "bioptic telescopes." But many older drivers don't use them, according to a linked editorial by Gordon Legge, head of a lab for low-vision research at the University of Minnesota in Minneapolis.
"For reasons that are not yet clear, few people with macular degeneration actually use bioptic telescopes," wrote Legge.