Countering Driver Vigilance Decrement for Partially Automated Vehicles

Current driving automation systems at SAE Level 2 automation (SAE, 2021) still require human drivers to monitor the driving scene to look out for unexpected problems in the roadway. Monitoring the roadway while the automation does much of the primary driving task quickly becomes monotonous and difficult over time. Drivers must maintain a vigil to watch for any potential issues, which is a difficult task for humans (Greenlee et al., 2018; Körber et al., 2015). Drivers tend to respond more slowly and make fewer safe responses to dangerous events as time goes on in a driving task, resulting in the vigilance decrement (Mackworth, 1948; Parasuraman, 1986). The vigilance requirement for human drivers of these partially automated vehicles can lead to decreased safety. Our study examined this vigilance decrement issue in the context of partially automated driving and develop ways to mitigate the decrement through various secondary tasks. For this study, we ran 117 participants using E-prime 3.0 with videos of a simulated driving environment created in STISIM. Participants were told they would be assisting an automated vehicle with detecting potentially dangerous situations in a roadway environment. The participants saw a steady stream of cars came towards them in the opposite lane. A practice drive ensured they could reliably detect the potential hazardous events. Over the 45-minute experimental drive, 68 of the 270 vehicles crossed over the center line before returning to their lane and participants were to press the space bar to indicate the event. The between-subjects independent variable was the vigilance-intervention strategy used – non-driving related (NDR), driving related (DR), or control. The intervention strategy was intended to either increase their general arousal (NDR) or prompt them to look at the driving environment (DR). NDR tasks asked simple knowledge questions, such as if January was the first month of the year, which could provide and attentional break but did not direct their attention. Whereas DR tasks asked about objects in the environment, such as if the current speed limit was 55mph, which directed their attention to specific objects but could increase their workload by splitting attention. The dependent variables include participants’ response times (RT) identifying the hazardous events, as well as the corresponding hit rates and false alarms. Additionally, the NASA-TLX (Hart & Staveland, 1988) workload measure the Short Stress State Questionnaire (Helton, 2004) were used to see differences before and after the study. The results for the RT and hit rate showed an increase in RT and significant decrease in hit rate for the DR and control conditions compared to the NDR condition, indicating that vigilance performance was better (more events were detected, and detection was faster) in the NDR condition. Previous research has shown similar results and has been used to support an explanation of attentional resource depletion (Greenlee et al., 2018, 2019; See et al., 1995). Overall, focusing on a task for too long depletes attentional resources resulting in decreased performance. Similar to the current study’s interventions, Ariga and colleagues (2011) showed a benefit of brief breaks to improve focus. Others demonstrated reduced vigilance decrement for participants who heard music at different intervals during the vigilance task (Davies et al., 1973). It is likely that the break can switch attention to a new task which breaks up the long period of goal habituation and allows for restrengthening the activation level. However, if it were simply a matter of attentional breaks because of mindlessness or underload, one would expect similar results for the DR and NDR conditions (Manly et al., 1999). The NDR condition showed numerically lower scores on the NASA-TLX (Hart & Staveland, 1988) workload measure and less engagement loss on the Short Stress State Questionnaire (Helton, 2004), providing further evidence for overload.