Influence of Taillight Width on the Ability to Recognize Closing Speed, Closing Distance, and Closing versus Separating

Front-to-rear crashes account for a large number of fatal collisions in the United States. While many of these crashes might be related to driver error (i.e. following too closely, inattention to traffic ahead, etc.) a cluster of these crashes are likely to be related to the human visual system limitations of depth perception during motion. Observers with valid CDL and non-commercial licenses were shown two 4-second video clips showing a slower moving vehicle ahead, referred to as the lead vehicle. The lead vehicle was depicted at distances of 91 m (300 ft) to 457 m (1500 ft) while closing at 72 km/h (45 mph). The lead vehicle was depicted on an unilluminated two-lane highway at night to allow the taillights to be the salient stimulus. The lead vehicle had either the standard taillights with a width of 1.7 m (5.4 ft) or narrowed taillights that were 0.4 m (1.43 ft) apart. The order in which each clip was viewed was counterbalanced. Observers consistently believed the narrower taillight configuration was farther away despite the vehicles’ headlights being on, allowing the entire vehicle width to be seen at distances closer than 128 m (420 ft). Also, observers perceived the wider taillight vehicle to be closing faster when viewing at distances closer than 128 m (420 ft). Drivers with CDL licenses performed no better or worse than non-commercial drivers which supports the hypothesis that crashes involving a high-speed vehicle closing on a slow moving or stopped vehicle might be related to human limitations, rather than driving experience, inattention or careless behavior.