A twelve month study of all accidends involving air-brake equipped motor vehicles in Southwestern Ontario was implemented to establish a data base from which a cost effective analysis could be performed to evaluate the need for improved Canadian Motor Vehicle Safety Standards (CMVSS) on trucks and buses. The study obtained specific accident data pertaining to the performance of existing air-brake equipped vehicles in an area patrolled by 25% of the Ontario Provincial Police manpower, approximately one-quarter of Ontario's roadways. This area allowed a study of criteria accidents in diverse traffic patterns and varying environmental conditions. During the study period a total of 27,813 reportable motor vehicle accidents occurred in the area. Of this total there were 827 (3%) accidents involving air-brake equipped motor vehicles. Methods employed during the data acquisition phase of the study ensured that all air-brake equipped motor vehicle accidents were recorded. Also, data on 86 alleged motor vehicle safety related defects was obtained.