An impaired operation detection method is basically performed by sensing a yaw rate of a vehicle over a time period. Then time points are identified in which the yaw rate changes direction. A linear rate of change in the yaw rate is determined between two of the time points that are sequential in time. An actual rate of change is examined between the two time points. A difference between the actual rate of change and the linear rate of change to obtain a difference value. Then, the operator's impairment condition is determined based on the difference value.