The document discusses measures of mental workload in drivers. It defines mental workload and provides a model with three hierarchical levels of driving tasks. Factors like the road environment, traffic, and vehicle ergonomics can affect driver workload. Various measures are described, including self-reports, primary task performance, secondary tasks, and physiological measures. Each measure has different characteristics in terms of sensitivity, reliability and ability to diagnose specific mental processes. Optimal driver performance occurs at low workload levels, and combining multiple measures provides the best assessment of a driver's mental workload.