This synopsis discusses the effect of diabetes on road safety. Diabetes mellitus is a group of metabolic diseases characterised by defects in insulin secretion, insulin action, or both. Studies generally show a (slightly) elevated risk for drivers with diabetes, although differences are often not statistically significant. Two main approaches have been used to study the relationship between diabetes and crash risk. The most common approach compares crash rates of individuals with diabetes with crash rates of individuals without diabetes. The less common approach first distinguishes between drivers who have and who have not been involved in a crash, and then compares the prevalence of diabetes in these two groups. Most research has been done in the USA, Canada, and Europe. Most of the research is on private drivers; very few studies are on commercial drivers.