Adopting a simplified linear model of an accident's timeline, the most frequent 'critical pre-crash event' cause is driver inattention. The 'critical pre-crash event' is defined as the action or event that puts the vehicle on an inevitable collision course - the point of no return.
Traveling to fast for the conditions was the critical pre-crash event in only 5% of cases, according to the study. The NHTSA says that 41% of all driver-related critical events were recognition errors related to inattention and internal or external distractions. Only about 8.4% of the driver-related critical events were related to excessive speed, however. By comparison, inadequate driver skill was deemed responsible for 10% of accidents.
Other prominent causes of accidents include turning or crossing at an intersection (36%), driving off the edge of the road (22%) or driving outside the lane (10.6%). A unique aspect of the study's conduct is that it involved investigators actually heading out to the accident scenes before they had been cleared by emergency personnel and recording data to determine the causation and timeline of the accident.
The remainder of the accidents were attributed either to the vehicles or roadway conditions themselves, rather than driver fault.