Traffic accident investigations have shown that some form of attentional failure is a common causal factor in traffic accidents. An overview of the literature suggested four basic attributes, or dimensions, of attentional performance:​ intensity;​ distribution;​ regularity;​ and mode of control. The indicators of attentional performance have been discussed in two major groups, behavioral and physiological. Behavioral indicators include:​ drivers' self-assessments of their attentional condition, observers' judgments of attention-related driving errors, driver-vehicle performance as measured through vehicle dynamics, drivers' looking behavior, and performance on non-driving tasks. Physiological indicators include such measures as heart rate, electrodermal response, and brain and muscle electrical activity. No single measure has been identified which reliably and unequivocally indicates inattention. Several issues for further research have been identified.