Driver's mental workload caused by the visual and auditory cognition has recently increased with the rapid spread of car navigation systems and other in-vehicle information devices and with the progress of Intelligent Transport Systems (ITS). Under such circumstances, the accurate evaluation of the workload is important in order to develop in-vehicle information devices that do not menace the safety.

In this paper, we present a basic idea to estimate the workload based on the driver's cognition, and show some experimental results for the estimation. The experimental data provide subjective, behavioral and physiological aspects of the workload, and clarify the quantification measure of the workload. Especially, we emphasize that event-related potentials and monitoring cerebral blood circulation are the most important evaluation indices to directly interpret the issues of driver's cognitive process and attention.
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