Electrodermal responses and verbal risk estimates as indicators of perceived level of risk during a car-driving task.

Several publications report the use of Electrodermal Activity (EDA) as a measure of perceived risk in the traffic environment. The experiment described in this report showed, however, that there are some practical and theoretical problems concerning the use of this measure in the domain of applied traffic research. Besides support of prior findings (e.g., Hulbert, 1957; Michaels, 1960 and Taylor, 1964), in-depth analyses of the EDA measures showed that the Electrodermal Activity does not necessarily reflect the risk perceived by the automobile driver. First, it turned out that 50% of the electrodermal responses (i.e., short lasting changes in the EDA) could not clearly be related to traffic events. Furthermore, the relationship between electrodermal responses (EDRs) and decelerations suggests that the motor activity that is associated with the decelerations could have played a role in the EDR elicitation. Especially traffic events that are associated with major bodily movements (e.g., changing of traffic lane) elicit both large decelerations and relatively large EDRs. These problems concerning the use of the EDA in traffic research can be summarized by saying that the EDA seems very sensitive to various kinds of stimulation, including motor behaviour. As a result, it is not very specific to changes in the perceived level of risk. Another risk-measure was the driver's verbal rating estimate of the perceived level of risk. This measure was sensitive to large experimentally induced variations in time headway (a decline in the time headway was linearly related to a rise in the mean verbal risk estimate). Minor changes in the time headway that were caused by changes in the traffic configuration did not systematically result in changes in the verbal risk estimates. A comparison of the verbal risk estimates with the EDRs supported our idea of the EDA not being very specific to changes in the perceived level of risk: traffic events that cause a rise in verbal risk estimate are most often associated with an EDR, while traffic events associated with an EDR only sporadically cause a rise in verbal risk estimate. For the covering abstract see IRRD 870346.