1. A method of assessing a head pose of a vehicle driver, comprising the steps of: measuring relative lateral movements of a head of the vehicle driver with a relative position sensor responsive to a posterior portion of the head; acquiring the measured relative lateral movements from the relative position sensor with a signal processor; processing the measured relative lateral movements with the signal processor to infer an apparent direction of said head pose by tracking the relative lateral movements and identifying lateral head movement behavior characteristic of a forward head pose, and to produce an output indicating whether said apparent direction is forward or non-forward with respect to a vehicle frame of reference; adjusting said apparent direction toward a forward-looking direction when lateral head movement behavior characteristic of said forward head pose is identified; and summing the measured relative lateral movements over a specified interval; and identifying lateral head movement behavior characteristic of said forward head pose when the summed movements are less than a calibrated threshold for at least a predetermined time.