The accident triangle, also known as Heinrich’s triangle or Bird’s triangle, is a theory of industrial accident prevention. It shows a relationship between serious accidents, minor accidents and near misses. This idea proposes that if the number of minor accidents is reduced then there will be a corresponding fall in the number of serious accidents. The triangle was first proposed by Herbert William Heinrich in 1931 and has since been updated and expanded upon by other writers, notably Frank E. Bird. It is often shown pictorially as a triangle or pyramid and has been described as a cornerstone of 20th century workplace health and safety philosophy. In recent times it has come under criticism over the values allocated to each category of accident and for focusing only on the reduction in minor injuries.

The triangle shows a relationship between the number of accidents resulting in serious injury, minor injuries or no injuries. The relationship was first proposed in 1931 by Herbert William Heinrich in his Industrial Accident Prevention: A Scientific Approach. Heinrich was a pioneer in the field of workplace health and safety. He worked as an assistant superintendent for an insurance company and wanted to reduce the number of serious industrial accidents. He commenced a study of more than 75,000 accident reports from the insurance company’s files as well as records held by individual industry sites. From this data he proposed a relationship of one major injury accident to 29 minor injury accidents, to 300 no-injury accidents. He drew the conclusion that, by reducing the number of minor accidents, industrial companies would see a correlating fall in the number of major accidents. The relationship is often shown pictorially in the form of a triangle or pyramid. The pyramid allows individuals to narrow down the root cause and eliminate or control the hazard or cause. The triangle was widely used in industrial health and safety programs over the following 80 years and was described as a cornerstone of health and safety philosophy. Heinrich’s theory also suggested that 88% of all accidents were caused by a human decision to carry out an unsafe act.