An attempt is made to explain the frequently observed law of accident frequencies (rising branch, peak value, descending branch) with its associated law of decreasing risk with increasing mobility, which itself develops approximately linearly with time. The search for accident potential-defining variables is reviewed and the components of the classical risk triad are considered. Three tentative explanations are discussed, respectively based on the concept of critical density, on the construction of new motorways and on a "dimensional" approach deduced from a physical analogy. The paper continues with the implementation of a friction model. A "behavioural" lognormal model is examined as an alternative explanation and an empirical law, inspired by Pearsons's frequency curves, is suggested as a purely descriptive tool. (A)
Abstract