Final results of a Tri-Level Accident Causation Study are reported, as well as results of special analysis projects investigating the relationship of driver vision, knowledge, and psychological attributes to accident involvement. A total of 2,258 accidents investigated by technicians, and 420 accidents investigated by a multidisciplinary team are reported. Human factors were cited as probable causes in 93% of accidents, compared to 34% for environmental factors and 13% for vehicular factors. Leading human factors included improper lookout, excessive speed, inattention, and improper evasive action. View obstructions and slick roads were leading environmental factors. The most frequently-involved vehicle factors were gross brake failure, inadequate tread depth, side-to-side brake imbalance, and underinflation. Vision (especially poor dynamic visual acuity) and personality (especially poor personal and social adjustment) were related to accidents. However, knowledge of the driving task was not shown to be related.