Driver Workload Defined

The literature indicates that task workload, separate from a participant’s abilities, depends on time, task difficulty, and structural interference between concurrent tasks.

Time is fundamental to the concept of workload. A dictionary definition of workload is “the number of hours that a machine, worker, teacher, etc., is required to work in any specific period” (Random House, 1969). Conceptual definitions of “workload as proportional to the ratio of time occupied performing tasks to total time available” also emphasize time as a key component of performance prediction (Wickens and Hollands, 2000).

Task difficulty is often used to describe any task modification that increases the required task time or decreases the accuracy of task completion (Kantowitz, 1987). Sarno and Wickens (1995) point out that more-difficult tasks usually take more time and so will predict greater interference between concurrent tasks. Thus, task difficulty may generally be addressed in reference to time. “More-difficult tasks take longer” is a useful generalization, but there are exceptions. For instance, difficult tasks might be completed faster than easy tasks if more errors are accepted. This is an example of a speed-accuracy tradeoff (Drury, 1999). Another exception is that long tasks made up of many simple activities may be less demanding than short tasks made up of fewer, more complex activities or processes (cf., Kantowitz, 1985). A third exception is that two tasks of the same duration can have different effects, e.g., just driving for two minutes versus two minutes of destination entry with a complicated route guidance system. Thus, caution should be used in driver workload data interpretation. Several steps may help in that interpretation. Task analysis can provide insights into the nature of the tasks being evaluated. Review of prior research, theory, and modeling also provide guidance. Unsuccessful task performances might be omitted or separated from the analysis of successful task performances. The distraction potential of a task, even if it is a long but monotonous or simple one, might also be assessed in terms of its demands relative to the concurrent demands of driving.

Task duration may be augmented, as needed, by a consideration of structural or resource interference between concurrent tasks (Groeger, 2000; Wickens and Hollands, 2000). The notion of structural or resource interference is based on basic human limitations. Two concurrent visual tasks cannot share foveal vision. Two concurrent auditory-vocal tasks cannot readily share listening and speaking resources. Concurrent tasks that load the same working memory resources can degrade performance on one or both tasks. Resources must be switched from task to task. Less interference is predicted when different input, central processing, or output resources are required by different concurrent tasks. Multiple resource theory is discussed in more detail in the DWM Task 1 report.

Chapter 1 Introduction

Visual-manual tasks and auditory-vocal tasks are fundamentally different in their resource demands from concurrent driving. Driving requires visual inputs to monitor the road scene; spatial working memory to perceive the position, speed, and acceleration of one’s own vehicle and others; and manual outputs to adjust steering, accelerator, and brakes. Verbal working memory is also required from time-to-time to read road signs, billboards, bumper stickers, and the like. Visual-manual tasks at a minimum require the same input and output resources as driving. Working memory demands may also overlap. Subsidiary task completion time reflects the duration of resource competition between visual-manual tasks and driving.

Auditory-vocal tasks require auditory inputs, vocal outputs, and (usually) verbal working memory. This implies relatively less structural interference with the driving task. As such, the duration of an auditory-vocal task may have little to do with intrusion on the driving task. A task performed with an auditory-vocal interface may take even longer than the same task with a visual-manual interface and yet it loads the driver less. The lower competition between input- output resources for auditory-vocal tasks and concurrent driving may leave certain aspects of the driving task unperturbed. Working memory demands, on the other hand, may leave at least some aspects of vehicle control unaffected but degrade object and event detection (Brown, 1994). Heightened emotional states can also lead to reduced situational awareness of vehicle control. However, this effect was not addressed in the DWM project.

Concurrent tasks performed while driving may compete with the primary driving task. Tijerina (1996) defined driver workload as the competition between subsidiary tasks and concurrent driving. The driver’s primary task is to safely control the vehicle at all times. Safe driving requires the driver to watch the driving scene, steer, manage speed and separation with other vehicles, and detect objects and events in the driving environment in order to respond as appropriate. These aspects of driving define the categories of workload measurement in a driving context.

To summarize the previous points into a definition of workload:

Workload, in the context of driver distraction, is defined as the competition in driver resources (perceptual, cognitive, physical) between the driving task and a concurrent subsidiary task, occurring over the task’s duration, as manifested in degraded lanekeeping, longitudinal control, object-and-event detection, or eye- glance behavior. For the purposes of this research, the workload occurs over the duration of the subsidiary task.

There is no validated transfer function that precisely relates workload measures to crash incidence. Studies that relate selected driver workload measures to crash incidence (e.g., Wierwille and Tijerina, 1998) are best treated as monotonic relations. The basic DWM strategy was to identify and evaluate measures thought to be monotonically related to quality of driving. This means that quality of driving should remain the same or decline as workload increases. The monotone relationship implies that quality of driving should not improve over a practical range as workload increases. This leads to the following relative interpretations when comparing higher- workload to lower-workload in-vehicle tasks:

• More erratic lanekeeping (greater weaving in the lane, more frequent departures out of lane during a task) reflects potentially worse, not better, lateral control while performing a task.

• Greater variation in speed or car following reflects potentially worse, not better, longitudinal control while performing a task.

• More misses reflect potentially worse, not better, object-and-event detection while performing a task.

Chapter 1 Introduction

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