Another important limitation of the study is that the measures of driving performance we were able to code and analyze – hands on steering wheel, direction of eye gaze, and vehicle wanderings or encroachments across travel lanes – have not been directly linked to crash risk. While we may intuitively feel that drivers who engage in activities that require them to take their hands off the steering wheel or their eyes off the road for short periods of time have a higher risk of crashing, we do not know this to be true. Neither do we know that increased wandering in the travel lane is associated with higher crash risks in real world driving.
Most importantly, we were unable to capture any measure of cognitive distraction, which has been linked in the literature to poorer driving performance and increased likelihood of crashing. Such studies have typically been carried out in more controlled settings, using driving simulators or instrumented vehicles (or drivers) on test tracks (see, e.g., Strayer, Drews and Johnston, 2003). Other studies have suggested that drivers’ fixed gaze may be an indicator of cognitive distraction. These studies distinguish two types of eye movements that can indicate a driver is distracted: either short glances away from the driving task, or the longer fixed gazes that signify a cognitive distraction. In our less controlled naturalistic driving study, we could not differentiate fixed gazes from the desirable category of “eyes directed at the roadway,” for example, when subjects were talking on a cell phone. Consequently, our study is not able to provide a definitive answer as to which activities, or which driver distractions, carry the greatest risks of crash involvement.
