Looming Threshold Limits and Their Use in Forensic Practice

Abstract

Rear-end collisions account for about one-third of all traffic crashes on a yearly basis. It has been apparent for some time that the key to perceiving a slower vehicle ahead of us as an imminent hazard, i.e., one requiring immediate action, is a sufficient rate of change of the visual angle of the lead vehicle. This is known as the “looming threshold”, among other things. A number of laboratory researchers have reported values of the looming threshold to be in the range of 0.003 radian/sec. Forensic practitioners routinely use elevated values of the looming threshold, e.g., 0.005-0.008, to account for the complexity of real-world driving tasks. However, only one source has used data from actual vehicle accidents to arrive at a looming threshold – and that value, 0.0275 rad/sec, is an order of magnitude larger than that derived from laboratory studies. In this study, we examine a much broader range of real-world accident data to obtain an estimate of the reasonable upper end of the looming threshold. The results show a range of 0.0397 to 0.0117 rad/sec, for presumed perception-reaction times (PRTs) of 0.75 and 2.0 seconds, respectively. We discuss the implications for forensic human factors analysis of rear-end crashes.