How to Determine the Time-Intervals of Two-Stage Warning Systems in Different Traffic Densities? An Investigation on the Takeover Process in Automated Driving

In level 3 automated driving, the warning systems played a critical role in reminding drivers who were engaging in non-driving-related tasks to take over the vehicle when the automated driving system met difficulties that exceed its capacity. The two-stage warning systems, which provided a time-interval between the first and the second warnings, compensated for deficiencies of the single-stage warning systems in inadequate motor preparation and situation awareness. However, the effectiveness of different lengths of time-intervals in different traffic densities remains unclear. The present study conducted an experiment to investigate different time-intervals (3 s, 5 s, 7 s, and 9 s) and traffic density levels (low: 0 vehicle /km and high: 20 vehicles/ km) on the drivers' motor readiness, situation awareness, takeover performance, and subjective ratings. Results suggested that 5 s and 7 s (compared with 3 s and 9 s) were two favored time-intervals in general but the drivers' response patterns and the beneficial aspects of time-intervals varied for driving environments with different traffic density levels. The findings of the present study had implications for the design and application of the two-stage warning systems.