How does drivers' attention change when using a two-stage warning system? Effects of expectation and cognitive load.

The advantages of two-stage warnings have been validated. This study investigated how drivers' expectations of automated driving system capabilities and cognitive load affect their attention allocation and takeover performance when using a two-stage warning system in a Level 3 automated driving system. Thirty-two drivers participated in a driving simulation study. The results showed that drivers under high cognitive load had longer and more frequent fixation on the road, which suggested a cautious attention strategy. The high-expectation group gazed less on the road and got greater lateral deviation and maximum acceleration. Attention allocation of the high-expectation group was similar between warning stages but was more susceptible to cognitive load within the same stage. The two-stage warnings need to be designed to direct drivers' attention effectively.