Traffic Safety for Adults with Diabetes

Diabetes is a common disease that may impair an adult's ability to drive a vehicle. As such, transportation regulators require documentation of good blood sugar control for drivers who wish to maintain a driving license. 

We studied consecutive drivers reported to the Ontario Ministry of Transportation (n = 795) who had a diagnosis of diabetes, reported their blood sugar control with HbA1c, or had documentation of a subsequent traffic crash. We found one-in-fourteen had been involved in a crash. Contrary to legal theories, those with better blood sugar control had a higher risk of a crash. The attributable risk was substantial (almost 1/2 the crashes), and mostly due to those who had severe hypoglycemic attacks. The adverse association persisted across a range of HbA1c values where the risk at the bottom quartile was twice the risk at the top quarter. This risk occurred regardless of treatment with injection or pills. Older patient age was associated with the greatest absolute risks. 

The main conclusion was that tighter glycemic control, as measured by the HbA1c, is associated with an increased risk of a traffic crash. One explanation is that intensive treatments sometimes causes low blood sugars that harm the brain or nerves and that make a person less fit to drive. This finding called into question laws in Ontario that restricted driver licenses on the basis of this measure of control. In turn, we subsequently worked with the Ontario Ministry of Transportation to highlight the difficulty in judging fitness-to-drive for adults with severe diabetes mellitus. A patient's HbA1c level is neither necessary nor sufficient. This has now resulted in a more thoughtful and holistic approach to assessing fitness to drive in Ontario and beyond.

Together, the work shows how science can inform traffic safety and guide laws that make roads safer for us all. More generally, the research is another example where science helps society learn from mistakes to lessen future suffering.