An accuracy analysis of techniques for measuring the durations of in-car manual tasks

Human factors research of automobile driver behavior often calls for timing in-car manual tasks. By measuring the time that a driver directs resources away from the primary task of driving, investigators can determine both if and where improvements need to be made in the driver-vehicle relationship. The present study was designed to compare the accuracy of various techniques for measuring in-car manual task durations. Additionally, this research was intended to reveal how closely these techniques approach the accuracy of the frame-by-frame video analysis method, which is time-consuming and expensive to perform.

The study required three male and three female subjects to use an electronic stopwatch to measure "hand-off-wheel" times for 30 driver tasks. Each subject performed this procedure three times: once while sitting as an observer in the back seat of a research vehicle, once while watching a real-time video recording of task performance, and once while watching a one-sixth real-time video recording of task performance.

Three independent variables were examined: timing Method (three levels), Duration of in-car task (three levels), and Subject (six levels). Dependent measures gathered were raw timing error (a measure of response bias), absolute timing error (a measure of response accuracy), and squared timing error (a measure of response consistency). Timing error was obtained by subtracting the measured time for a particular task from the "true" task time obtained by using the frame-by-frame video analysis technique.

Analysis of the data indicated a significant effect of Method on response bias. Specifically, use of the slow-motion video technique resulted in overestimation of in-car task durations, and use of the two real-time techniques resulted in estimates of task durations that were either equal to or less than the true durations. Significant effects of Subject, Gender, and Subject x Method were also revealed. The results suggest that the on-road timing technique should be used in the future, since this procedure requires little in terms of cost and implementation time, and errors are small when compared with the frame-by-frame technique. Furthermore, it is recommended that only one experimenter serve as a timing analyst during an experiment to ensure a constant level of accuracy.