This study aimed to reveal the effects of modality of warnings and secondary tasks in response to visual hazards in driving situations and to examine the design principles of a collision warning system. This study investigated participants’ driving performance and hazard detection performance as they carried out a secondary task. The experimental factors were whether a driver performed a secondary task, the modality of the secondary task (i.e., visual, or auditory), and the type of warning (i.e., none, visual, auditory, or tactile). The participants were asked to simultaneously carry out a simulated driving task, a secondary task, and a hazard detection task. Their reaction times to frontward hazards were significantly long when the modality of a warning and that of a secondary task overlapped. Moreover, there was no disadvantage of the tactile warning in other performance measurements. These results indicate that a tactile warning could be the most effective modality for a collision warning system because it does not overlap with the modality of driving or with visual and auditory secondary tasks.
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