Following the fire in the Mont Blanc tunnel, a new regulation was passed requiring each tunnel of a study of its specific risks and, for the transit of dangerous goods, a comparison with the risks alternative routes. In the practical implementation of these new studies, the temptation was great to multiply and more sophisticated modeling of fires, scenario analysis, the probability calculations, the criteria for making rational choices ... Group working to develop good practices, which had both an extensive field experience and scientific instruments, has gradually converged to a specific doctrine of "reasonable use" of science by their place in the traditional arguments based on simple rules, but tough. Be described by the complex trade-offs made ​​between two logics, the logic "hyperrationnelle" and the pragmatic logic, which are always more or less tension in the regulation of risk.