AbstractThe purpose of this paper is to demonstrate how the relation between the mean arrival rate of break-downs conditional upon the inspection frequency of equipment can be estimated in practice. The work was carried out for a large, urban transit authority operating a fleet of approximately 2000 buses undertaking about 80 million kilometres per year. A model relating total downtime of buses incurred due to inspections and repairs per unit time to inspection frequency is developed, and the optimal inspection frequency which maximizes bus availability is determined.