The Risk of a Safety-critical Event Associated with Mobile Device Subtasks in Specific Driving Contexts

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National Surface Transportation Safety Center for Excellence


Using a cell phone while driving has been associated with an increased crash risk. However, when examining the inherent subtasks, naturalistic driving studies (NDSs) have shown that safety-critical event (SCE) risk is associated with visual-manual subtasks – which take the driver’s eyes off the roadway – and not the conversation itself. This study consisted of an analysis of data from NDSs involving commercial motor vehicle (CMV) drivers and light vehicle (LV) drivers. The NDS data sets were partitioned into subsets representative of specific driving contexts. Groupings of (1) level of service, an ordinal measure of traffic density, (2) relation to intersections and merge ramps, and (3) combinations of these two factors were prepared. The data were then “flagged” whenever specific mobile device subtasks were observed. Mobile device usage was investigated by summing all SCEs and baselines for which a subtask was observed in each context and comparing the likelihood of the subtask occurring in each context using chi-squared tests. Odds ratios and their respective 95% confidence intervals were then computed for mobile device subtasks in each context to investigate the association between their presence and the occurrence of an SCE. It was found that CMV and LV drivers varied as to how much they conversed on a mobile device, but did not vary their engagement in visual-manual subtasks, across the driving contexts examined. Furthermore, CMV drivers conversed less frequently when driving task demands were great, and LV drivers did not. The risk of an SCE associated with mobile device use (collapsed across subtasks) was dependent on the driving context as well as each subtask’s associated SCE risk. Only visual-manual subtasks were associated with an increased SCE risk, while conversing was associated with a decreased risk. Overall, the study shows that drivers’ engagement in mobile device subtasks, and the associated SCE risk, varies by driving context. The findings can be used to inform the design of in-vehicle interfaces that mitigate distraction by preventing visual-manual subtasks while driving.



Cell phone use, Crash risk, Safety critical events, Naturalistic driving, 100-car, Distracted driving