This paper provides final results of an accident investigation study conducted by the Road Safety Team of École Polytechnique de Montréal, for the Société de l'Assurance Automobile du Québec (SAAQ). Particular emphasis is placed on the role of mechanical condition of trucks and tractor trailers in accident causation. These results are put in context with the evaluation of the impact of the annual Mandatory Mechanical Inspection Program (MMIP) carried out in the province of Québec, by analyzing the nature and frequency of defects. The relation between age and mechanical condition is discussed. Air-brake system efficiency and maintenance is also addressed. Furthermore an indicator of accident-causing potential is defined for each major mechanical component, based on their contribution to the investigated accidents.