Combining cognitive and visual distraction: less than the sum of its parts.

Driver distraction has become a leading cause of motor-vehicle crashes. Although visual and cognitive distraction has been studied extensively, relatively little research has addressed their combined effects on drivers' behavior. To fill this gap, a medium-fidelity simulator study examined the driver behavior before, during and after three types of distraction. Driving without distraction was compared to visual distraction, cognitive distraction, and combined visual and cognitive distraction. The results show that the visual and combined distraction both impaired vehicle control and hazard detection and resulted in frequent, long off-road glances. The combined distraction was less detrimental than visual distraction alone. Cognitive distraction made steering less smooth, but improved lane maintenance. All distractions caused gaze concentration and slow saccades when drivers looked at the roadway, and cognitive and combined distraction increased blink frequency. Steering neglect, under-compensation, and over-compensation were three typical steering failures that were differentially associated with the different distractions: steering neglect and over-compensation with visual distraction and under-compensation with cognitive distraction. Overall, visual distraction interferes with driving performance more than cognitive distraction, and visual distraction dominates the performance decrements during combined distraction. These results suggest that minimizing visual demand is particularly important in the design of in-vehicle systems and in the development of distraction countermeasures.