Impact of free-choice non-driving related activities on driver fatigue and performance in the context of conditional automated driving

A critical safety risk during conditional automated driving (CAD, SAE Level 3) is driver fatigue. Researchers have investigated several countermeasures to address this issue. Non-driving related activities (NDRAs) during CAD have shown considerable promise in preventing driver fatigue and should be evaluated in more detail. The aim of this study was to investigate free-choice NDRAs as a potential countermeasure against driver fatigue. For this purpose, 78 participants took part in a driving simulator experiment with a 2x2 mixed design. The participants drove manually before and after CAD to compare the impact of driver fatigue on driving performance. During CAD, the participants were randomly assigned to one of two conditions: with vs. without an NDRA. The participants of the first condition were allowed to engage in NDRAs after reaching a 7 on the Karolinska Sleepiness Scale (KSS) during CAD. Driver fatigue, takeover performance, and driving performance were assessed. The results revealed that driver fatigue was significantly lower during CAD in the group with an NDRA than without. However, the NDRA had no significant effect on driving performance (i.e., distance headway, time-to-collision, standard deviation of lateral position) in the second manual drive. Driver fatigue resulted in significantly worse driving performance in the second drive. The results showed that an NDRA only impacted driver fatigue during CAD, not manual driving performance afterwards.