Task priority management

Under high task demands, drivers may not be able to perform all tasks simultane- ously, but have to drop certain tasks as a consequence: “Attention implies withdrawal from some things in order to deal effectively with others“ (James, 1890/1950, p. 403- 404). A disastrous example of when such withdrawal of attention failed in order to deal effectively with a more important task is the Eastern Airlines L1011 crash into the Everglades in 1972, where the flight crew, preoccupied with a landing gear prob- lem, failed to monitor their altitude (NTSB, 1973). All three crew members and a jump seat occupant became totally absorbed in the diagnosis task. According to Granda et al. (1991), significant altitude deviations resulting from task neglect are a major con- cern in the aviation industry because of their growing frequency of occurrence. Con- sequently, distraction not only describes a lack of attention but also the act of attend- ing to something irrelevant with the result of an impaired capacity to process relevant information (Rumar, 1990). Therefore, effective task performance involves selecting relevant information while ignoring irrelevant information. But how appropriate is hu- man behaviour in selecting which task to do when?

Different components of a complex task have different functional priority with respect to the overall goals or temporal limits. One task can be highly relevant in one situa- tion and inappropriate in another, and thus, the driving context determines to a large extent the priority of a certain task. According to Funk (cited from Chou, Madhavan, & Funk, 1996, p. 308), strategic task management involves several components. First, the initiation of tasks when appropriate conditions exist – the speed with which activi- ties can be initiated depends on the degree of automaticity in that specific task and the degree of possible alternatives. Second, the assessment of task progress and status (task monitoring). Third, the assignment of priorities to tasks relative to their importance and urgency for the safe completion of the mission (task prioritization). Fourth, the assignment of human and machine resources to tasks so that they may be completed. Fifth, the temporary suspension of lower priority tasks, so that re- sources may be allocated to higher priority tasks, and the resumption of interrupted

tasks when priorities change or resources become available. And finally, the termina- tion of tasks that have been completed, that cannot be completed, or that are no longer relevant. Actions usually can be stopped rapidly, although under high stress there may be a tendency to inhibit action stopping or switching, i.e., in high stress situations, activities may persist longer than they should (Huey & Wickens, 1993). Task priority management therefore makes the assumption that the optimal task manager will process a mental priority scale that can provide the basis for appropri- ately shedding tasks when workload becomes excessive, so that the operator can address high-priority tasks before those of lower priority. There appears to be little data, however, to indicate the effectiveness of subjective priority in driving task man- agement in operational environments (Huey & Wickens, 1993). In a study by Cnossen et al. (2000), however, car drivers were found to reduce their driving speed only when primary task demands increased. Increases in secondary task demands resulted instead in skipping the subsidiary task when task demands increased be- yond capability or motivation of the participants. Cnossen et al. (2000) consequently argue that drivers prioritise their tasks with respect to the main task goal, which is to arrive safely at the destination. Therefore, tasks that serve the driving task directly (e.g. route guidance system information) will receive higher priority from drivers than tasks that are less important to the driving task (Cnossen, Meijman, & Rothengatter, 2004), such as changing the radio channel for example. Wierwille et al. (1991) found that drivers did adapt to high anticipated attentional demand by increasing the pro- portion of time spent looking at driving-related visual areas, while decreasing the proportion of time spent observing the navigation display by about the same amount. Contrary to these findings, Moray (1986), Jersild (1927), Sheridan (1972), Rogers and Monsell (1995), and Jamson and Merat (2005) reported a tendency to continue performing a lower-priority task longer than is optimal if the need to perform a higher- priority task arises. Similarly, Dingus et al. (1997) found that drivers had more un- planned lane deviations when driving with a complex route guidance system due to inappropriately long glances at the displays. Cnossen et al. (2004) reported similar effects: even when participants had already reduced their driving speed to counteract the negative effect of map reading, they still swerved more. Jameson and Merat (2005) found that participants seemed incapable of fully prioritising the primary driv- ing task over either a visual or cognitive secondary task.

As different components of a task may have different functional priority with changing situational contexts, one task can suddenly become highly relevant in a certain situa- tion. Therefore, effective task management depends highly on an optimal situation awareness. Knowing what tasks are currently in the queue that need to be done (Huey & Wickens, 1993) is crucial for prioritizing the currently most important task. Therefore the next paragraph will deal with situation awareness.