Assessing subjective criticality of take-over situations: Validation of two rating scales.

Assessing subjective criticality of take-over situations is crucial for understanding of take-over behavior and comparing studies. However, no validated rating scales exist that assess subjective criticality of take-over situations. In a driving simulator study, two rating scales, the Scale of Criticality Assessment of driving situations from Neukum et al. (2008) and the Criticality Rating Scale, were tested on their validity to assess the subjective criticality of take-over situations. Besides, the subjective and behavioral changes over the repeated experience of take-over situations were investigated. Twenty-five participants experienced a set of five take-over situations with varying time-to-collisions (TTC) at the moment of the take-over request, twice. After each of the first five take-over situations, participants rated the criticality on one scale, after each of the second five situations on the other scale. Correlation coefficients between TTCs and criticality ratings for each scale were calculated. Also, the changes of subjective and behavioral measures over the trials were investigated. Correlation coefficients indicated a strong correlation between criticality ratings and TTCs. Hence, both scales are equally valid for the assessment of the criticality of take-over situations. The repeated experience of the take-over situations did not affect effort ratings, take-over times, or steering wheel positions. But brake input decreased with increasing practice, indicating a safer take-over behavior. Hence, results of studies with repeated experience of take-over situations are relatively valid as only brake behavior changed with increasing practice.