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.