This study was conducted to determine the specific driver behaviors and unsafe driving acts that lead to crashes, and the situational, driver and vehicle characteristics associated with these behaviors. A sample of 723 crashes involving 1284 drivers was investigated from four different sites in the country during the period from April 1, 1996 through April 30, 1997. The crashes were selected using the National Automotive Sampling System (NASS) protocol and provide a fair sample of serious crashes involving passenger vehicles in the United States. In-depth data were collected and evaluated on the condition of the vehicles, the crash scene, roadway conditions, driver behaviors and situational factors at the time of the crash. Investigators used an 11 step process to evaluate the crash, determine the primary cause of each crash, and uncover contributing factors. Crash causes were attributed to either driver behavior or other causes. In 717 of the 723 crashes investigated (99%), a driver behavioral error caused or contributed to the crash. Of the 1284 drivers involved in these crashes, 732 drivers (57%) contributed in some way to the cause of their crashes. There were six causal factors associated with driver behaviors that occurred at relatively high frequencies for these drivers and accounted for most of the problem behaviors. They are: Driver Inattention - 22.7%, Vehicle Speed - 18.7%, Alcohol Impairment - 18.2%, Perceptual Errors (e.g., looked, but didn't see) - 15.1%, Decision Errors (e.g., turned with obstructed view) - 10.1%, and Incapacitation (e.g., fell asleep) - 6.4%. Problem types in terms of crash configuration and specific problem behaviors were also identified. The following seven crash problem types accounted for almost half of the crashes studied where there was a driver behavioral error: Same Direction, Rear End (Driver Inattention Factors) - 12.9%; Turn, Merge, Path Encroachment (Looked, Did Not See, etc.) - 12.0%; Single Driver, Roadside Departure (Speed, Alcohol) With Traction Loss - 5.5%; Single Driver, Roadside Departure Without Traction Loss - 4.8%; Intersecting Paths, Straight Paths (Looked, Did Not See, etc.) - 4.1%; Same Trafficway, Opposite Direction (Inattention, Speed) - 2.6%; and Backing, Other, Miscellaneous, etc. (Following Too Closely, Speed) - 1.3%. Countermeasures were identified in the areas of education, training, and law enforcement for each of the major problem types described in this effort. A limited number of technology based countermeasures related to the Intelligent Transportation Systems (ITS) initiative were also identified when these countermeasure types were relevant to specific driver error tendencies. A technical summary report which highlights major findings of this effort has been prepared and is available to the general public.