An analytical method for defining and describing the behavioral elements of those tasks that are inherent in the operation of large cargo trucks for mission durations ranging from four to eight hours was developed and validated in this study. The preliminary analysis of task elements was subjected to validation by several means: structured checklists and interviews with veteran truck drivers, en route observations, and activity analyses vis a vis such personnel, and by elicitation of driving "critical incident" reports. From the task data base, certain tasks were selected on the basis of their estimated (1) criticality to perational safety, (2) susceptibility to degradation as a function mission duration, and (3) "predictability," or the inverse of the proportion of occurrences that a given task is performed on a contingency or emergency basis. (Author/DB)