Secondary Task Measurement of Workload as a Function of Simulated Vehicle Dynamics and Driving Conditions

A driving simulator with a six-degree of freedom computer-generated display, a four degree of freedom physical motion system, and a three-channel sound system was used to determine the sensitivity of a secondary task to vehicle handling parameters and various driving conditions. Six subjects drove a simulated vehicle with normal automobile handling and another six drove with degraded handling (slow response). Steering ratio and disturbance level were adjusted within each set of six subjects. A secondary task consisting of reading random digits aloud from a single-digit dashboard display was used to assess workload. Using a technique similar to that of Knowles (1963) and McDonald (1973), it was found that workload increased significantly as disturbance level increased. Furthermore, workload increased significantly with degraded vehicle handling. In contrast, increasing steering ratio did not produce a significant change in workload. These results indicate that the secondary task method can be used to assess the major effects of simulated vehicle handling on driver workload. The secondary task used is directly transferable to test vehicles, and it allows the assessment of large changes in primary task difficulty even though direct primary task measurement may not be feasible or economical. Problems remain, however, in designing more sensitive secondary-task measures.