A method predicts and quantifies the threat posed to a human-operated device based on an optimal device trajectory through a constraint-bounded corridor. A model of the device together with a model of anticipated hazards and the current state of both the device and the hazards are used to iteratively generate an optimal device trajectory through a constraint-bounded corridor or region within state space. Device dynamics are forward-simulated over a time horizon. A method generates a threat assessment metric from the resulting sequence of optimal vehicle states. This threat assessment may be used to devise various types and levels of operator assistance. The human operator can control the device within a safe corridor or region. Threat assessment is based on the nearness of successive optimal trajectory predictions to limits of safe device handling rather than on deviation from a predefined path.