Driver’s cognitive load has always been associated with the driver’s heart rate activity and his/her skin conductance activity. However, what aspects of cognitive load that these signals relate to have never been clearly studied. This paper presents our preliminary results about the relationship between the different physiological signals (heart rate and skin conductance) and the driver’s cognitive load. Via one experiment with simulated car driving environment and one experiment in real flying environment, our data suggests that subjects’ heart rate relates to the number of events to be processed by the human driver while the skin conductance relates to the novelty of the driving task. Given the small population involved in these experiments, tests on more subjects are planned and reported in the future