Impact of the difficulty of the box task on its sensitivity when combined with a detection response task to assess secondary task demand while driving

Multimodal in-vehicle infotainment systems offer drivers a range of non-driving-related functions but can increase visual-manual and cognitive task demand, compromising road safety. Therefore, it is important to estimate the secondary task demand of these systems early in the development process. To do so, the Box Task combined with a Detection Response Task (BT + DRT) was developed as a straightforward laboratory method. The BT is used to quantify the visual-manual task demand, while the DRT is capable of assessing cognitive demand. However, previous studies showed that difficult cognitive secondary task demand led to a similar decrease in performance in the BT to that found in the easy visual-manual demand. Therefore, this study aimed to enhance the BT’s sensitivity to visual-manual demand and discriminability from cognitive demand by increasing the difficulty of the BT. Additionally, the effects of increased BT difficulty on DRT metrics, self-assessed mental workload and secondary task performance were examined. In total, N = 39 participants performed the BT + DRT with varying BT difficulty levels (easy, moderate and difficult), secondary task types (visual-manual vs. cognitive) and secondary task difficulty levels (easy vs. difficult). The results indicated that lateral variability at moderate and difficult BT levels was the BT metric with the largest and most consistent effect sizes for assessing visual-manual secondary task demand and to discriminate from performance impairments resulting from cognitive task demand. At both BT levels, the DRT is also capable of effectively assessing cognitive demand, either through response time or the number of omissions. For self-assessed workload, only slight increases in ratings were observed for higher BT difficulty levels. There were only minor changes in secondary task performance, such as slightly slower responses, during more difficult BT levels. Consequently, a higher BT difficulty than previously used is recommended for the BT + DRT paradigm.