Estimates of Driver Mental Workload: A Long-Term Field Trial of Two Subsidiary Tasks

Abstract

Two auditory subsidiary task measures of driver mental workload---delayed digit recall and random digit generation---were evaluated in a four-year field trial. Van pool members performed the tasks for 2-min periods while driving a mix of rural secondary roads, limited access expressways, high-density, limited-access urban drives, and downtown city streets on a daily commute from upstate New York to New York City. Data collected included the roadway being traveled, time of day, traffic conditions, vehicle density, speed, weather, brake applications, and driver's subjective difficulty rating. Subsidiary task degradation was a function of traffic density, average speed, and uncertainty (estimated by the number of brake depressions). Weather conditions moderated these variables. Unpredictability of traffic appeared to be the major determinant of perceived difficulty. The digit recall task correlated (r = 0.784) with driver ratings of task difficulty and (r = 0.808) with a calculated driver workload index based on brake actuations per minute plus the log2 of vehicle speed. Although subjective difficulty ratings correlated with subsidiary task performance and with objective criteria of traffic density and speed, some dissociation was noted. Indications of workload homeostasis were evident, as drivers appeared to modify their performance to keep workload within a comfortable range.