It’s often difficult for doctors to warn patients that they may no longer be fit to drive, but a new Canadian study finds that when they do, the result is fewer serious crashes.

The study is one of the first to look at how formal warnings from doctors affect the number of crashes on the roads. And it concludes that it often takes a warning from a doctor to convince an aging or unhealthy driver that it’s time to hang up the keys.

In Ontario, the Highway Traffic Act states that doctors must warn patients whom they think might be unfit to drive about their concerns. These drivers are not just those who are aging or whose eyes are failing them; it’s also patients with chronic conditions such epilepsy, alcoholism, uncontrolled diabetes or motor control problems.

In fact, chronic medical diseases are thought to contribute to about one-third of all road crashes.

Ideally, a doctor counsels patients about how their medical conditions make them a danger behind the wheel, and work with them to find alternative transportation. But some drivers dismiss the advice. That’s why doctors are also obligated to report these patients to the province’s Registrar of Motor Vehicles, which will suspend the patients’ drivers’ licences in about 10 to 30 per cent of cases.

But doctors are often reticent to discuss this issue with patients or overwhelmed with paperwork. So in 2006, Ontario come up with a new approach: it became the first province in Canada to offer doctors a financial incentive to report potentially dangerous drivers.

The province now pays doctors $36.25 as a fee for each patient they report, in the hope that it will save lives and cut costs for the entire medical system.

Now, a new study in the New England Journal of Medicine finds that the system is working.

The study tracked more than 100,000 patients who received medical warnings from their doctors between April 1, 2006 and March 31, 2010. They found that the warning led to a 45 per cent decrease in serious trauma from road crashes.

The decrease was most notable among elderly women with high socio-economic status, who were living in rural areas.

The authors of the study estimate that the medical warnings saved Ontario about $7 million each year by preventing crashes.

Dr. Donald Redelmeier, the lead investigator of the study and ICES senior scientist, says the drop in serious crashes was immediate.

“It is a big reduction that is profound, sustained and quite robust,” he told CTV News.

“Medical warnings from physicians about weight loss or exercise are not nearly as effective. So it is the combination of physician authority and government action that are powerful forces,” he says.

Many jurisdictions mandate that doctors report unfit drivers, but Ontario is the first to pay them to fill out the paper work.

“It's not that physicians are greedy, it's that they are so overwhelmed with multiple things to do. Once a nominal fee is in place it does draw people’s attention to the program” Redelmeier.

The study also noted that the driver warnings had other effects. In as many as 10 per cent of cases, patients did not return again to visit a doctor after they received a warning about their driving. As well, some patients later sought help for worsening depression.

They also note that even patients who have received warnings from their doctors but who did not have their licences suspended still had crashes at rates higher than the general population.

“The results of this study suggest that medical warnings may help to prevent trauma from road crashes. The data also suggest that incentives for physicians to issue such warnings increase their frequency,” the authors write. “The main risk of such practices is that, taken to the extreme, they could result in lost freedoms for patients who might be inherently safe drivers.”

Woodstock, Ont., resident Alan Burridge, 76, felt the program’s full effects. A doctor notified officials after he had been diagnosed with early stage dementia. Burridge thought he was a good driver, but he failed the driver’s test ordered by provincial officials as a result of the doctor’s warning.

“I was angry for a long time, no doubt about it,” Burridge told CTV News. “I reconciled to it now but certainly for some time initially I wasn’t and I probably wasn’t too good to live with.”

Burrdige’s wife, Maureen, now drives him.

“I was concerned he would get into an accident,” she said. “His reactions were that much slower within other traffic and yes, it caused me concern.”

Still, Burridge’s inability to drive has affected the couple “dreadfully,” Maureen said. She said it took away her husband’s self-esteem and left him dependent on others.

In fact, the study identified that the program can trigger or exacerbate depression in those who lose their licences -- a side-effect that authors say has to be watched for and possibly treated.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip