"We found that each warning led to about a 40 to 50 per cent decrease in the risk of a serious road crash for the patient," said principal researcher Dr. Donald Redelmeier, a senior scientist at the Institute for Clinical Evaluative Sciences.
"This decrease was immediate in onset, profound in magnitude and sustained in duration," said Redelmeier. "An effect of this magnitude is about two times larger than the combined effects of modern trauma hospitals on saving people's lives."
Chronic health conditions — such as dementia, strokes, alcoholism, epilepsy and diabetes —can make it risky for patients to take the driver's seat, he said, noting that drivers with such diagnoses contribute to about one-third of all traffic crashes.
"The majority of cases, about 75 per cent of individuals, are over the age of 65," he said. "But there are some individuals who are 45 and younger, most commonly with schizophrenia, alcoholism, substance abuse and attention deficit disorders. Those are the sort of common illnesses among the younger patients who were warned."
Since 1968, Ontario doctors have been required by law to report unfit drivers, but studies have shown that actual reporting of affected patients falls far below the prevalence of chronic diseases like dementia, alcoholism and diabetes in the population.
In 2006, physicians in the province began receiving a small fee — $36.25 — for counselling a patient that he or she should not be driving, and reporting them to the Ministry of Transportation.
Redelmeier and his team set out to see what effect a doctor's warning might have on the risk of having a serious road crash.
Their paper, published Wednesday in the New England Journal of Medicine, used provincial health records to identify more than 100,000 patients who had received medical cautions from more than 6,000 doctors between April 2006 and March 2010.
The researchers then evaluated each driver's need for emergency care to treat serious injury from a road accident in the four years prior to their warning and the year after. In all, these drivers were involved in about 1,700 serious traffic accidents.
They found there was a 45 per cent drop in severe trauma from motor vehicle crashes in the 12 months after patients were warned they could be courting disaster by getting behind the wheel.
In the four years before, there were 1,430 road crashes in which a patient was the driver and presented to the emergency department, compared with 273 crashes during the year after patients were counselled against driving.
Doctors' warnings don't always result in unfit drivers giving up their car keys, said Redelmeier. While 10 to 30 per cent end up having their licences suspended, most patients get "a rather stiff warning letter" from the Ministry of Transport, he said.
"So although the majority retain their licence, they lose their peace of mind. So on account of that, some of them probably stop driving, whereas others just stop driving as recklessly. They actually stop at stop signs and they yield the right of way ... and they don't speed."
Still, doctors' admonitions can also have some downsides, he conceded.
The study found that the rate of severe depression among patients was 25 per cent higher in the year after they were advised not to drive, based on emergency room records.
Researchers also found that an unfit-driver warning could erode the doctor-patient relationship.
"About one out of every five patients never makes a return visit to the physician responsible for giving the warning," said Redelmeier. "So they don't leave the system, but they do take their business elsewhere."
However, he said these are not good reasons to avoid telling patients they could be putting themselves, their passengers, other drivers and pedestrians in danger.
"It's not a reason to withhold the warning; instead, it's a reason for trying to provide the warning in a thoughtful and compassionate manner."
Researchers found a reluctance among physicians to warn patients, even those who had multiple conditions that would make them unfit to drive. Many had five or more diagnoses, he said.
As an emergency room physician, Redelmeier knows such conversations aren't easy.
"After you sort of settle their fractures and replace their blood and they seem to be recovering from their injuries, you have to get around to these conversations. And sometimes I wondered if I was doing any good. That was the motivation for doing the research.
"So I think it's a big affirmation that physicians really can make a difference when it comes to roadway safety."