Development and evaluation of new anchors for ratings of driving workload
Older drivers stop driving for several reasons, including being overwhelmed by the workload of the primary driving task. Unfortunately, most driving studies (including those measuring driver distraction and overload) describe workload qualitatively, not quantitatively. A simple way to quantify workload is to ask drivers to rate it while showing them anchor clips (e.g. this scene is 2, that scene is 6) to aid repeatability. To validate new anchor clips of road scenes for workload ratings, 16 subjects (8 age 18-30, 8 age >65) drove simulated expressway scenarios and rated the workload of 28 scenarios relative to the new anchor clips, and for 10 of them that duplicated video clips from real expressways, rated the video clips as well. Mean workload ratings from the 2 presentation methods were highly correlated (r=0.84). Workload ratings were correlated with the logarithm of the distance to the lead vehicle (r=-0.75), the number of vehicles visible (r=0.72), the distance to the side vehicles (r=-0.35), and lateral lane position (r=0.74). Workload can be estimated as 8.53-3.18*Log(Gap) + 0.28*MeanTrafficCount + 4.70*MinimumLanePosition - 0.10*StandardDeviationOfSideVehicleGap, with the R2 of 0.89. To refine the anchor clips, 18 subjects (6 age 18-30, 6 age 35-50, 6 age >65) were shown static scenes in which the field of view (120, 150, 180 degrees), and the rear scene (nothing, 3 mirrors, panoramic mirror) varied. Subjects recalled where vehicles were shown (2-11) and ranked scenes from most to least preferred. Scenes with 120- or 180-degree fields of view showing a rear scene are recommended for the anchors. Researchers are encouraged to quantify workload using the anchored rating method and the associated equations given their repeatability and ease of use.