This study examined the effects of visual-verbalload (as measured by a visually presented reading-memory task with three levels) on a visual/auditory stimulus-response task. The three levels of load were defined as follows: "No Load" meant no other stimuli were presented concurrently; "Free Load" meant that a letter (A, B, C, or D) appeared at the same time as the visual or auditory stimulus; and "Force Load" was the same as "Free Load," but the participants were also instructed to count how many times the letter A appeared. The stimulus-response task also had three levels: "irrelevant," "compatible," and "incompatible" spatial conditions. These required different key-pressing responses. The visual stimulus was a red ball presented either to the left or to the right of the display screen, and the auditory stimulus was a tone delivered from a position similar to that of the visual stimulus. Participants also processed an irrelevant stimulus. The results indicated that participants perceived auditory stimuli earlier than visual stimuli and reacted faster under stimulus-response compatible conditions. These results held even under a high visual-verbal load. These findings suggest the following guidelines for systems used in driving: an auditory source, appropriately compatible signal and manual-response positions, and a visually simplified background.

To investigate interference between the sustained and occasional attention required in driving, performance of 36 participants engaged in a focal manual tracking task and a peripheral detection task, representing sustained and occasional attention, respectively, was studied. Error ratio, tracking distance, tracking speed, and root mean squared tracking distance error were taken for manual tracking, and response times were measured on the detection task. Analysis indicated that multiple tasks or highly sustained attentional demands preceded lower performance on sustained attention tasks. Performance on occasional attention tasks after multiple tasks may also decrease, but occasional attention performance improved when the participants were engaged concurrently in a task involving higher sustained attention. Furthermore, the association between sustained and occasional attention was strengthened as the number of tasks or the demands of sustained attention increased.